[Why]
If we have a single MST display and we disconnect it, we dont disable that
link. This causes the old link settings to still exist

Now on a replug for MST we think its a link loss and will try to reallocate
mst payload which will fail, throwing warning below.

[  129.374192] [drm] Failed to updateMST allocation table forpipe idx:0
[  129.374206] ------------[ cut here ]------------
[  129.374284] WARNING: CPU: 14 PID: 1710 at
drivers/gpu/drm/amd/amdgpu/../dal-dev/dc/core/dc_link.c:3153
dc_link_allocate_mst_payload+0x1f7/0x220 [amdgpu]

[  129.374285] Modules linked in: amdgpu(OE) amd_iommu_v2 gpu_sched ttm
drm_kms_helper drm fb_sys_fops syscopyarea sysfillrect sysimgblt
binfmt_misc nls_iso8859_1 edac_mce_amd snd_hda_codec_realtek
snd_hda_codec_generic ledtrig_audio kvm snd_hda_codec_hdmi snd_hda_intel
snd_intel_nhlt snd_hda_codec irqbypass snd_hda_core snd_hwdep snd_pcm
snd_seq_midi snd_seq_midi_event snd_rawmidi crct10dif_pclmul snd_seq
crc32_pclmul ghash_clmulni_intel snd_seq_device snd_timer snd aesni_intel
eeepc_wmi crypto_simd asus_wmi joydev cryptd sparse_keymap input_leds
soundcore video glue_helper wmi_bmof mxm_wmi k10temp ccp mac_hid
sch_fq_codel parport_pc ppdev lp parport ip_tables x_tables autofs4
hid_generic usbhid hid igb i2c_algo_bit ahci dca i2c_piix4 libahci
gpio_amdpt wmi gpio_generic

[  129.374318] CPU: 14 PID: 1710 Comm: kworker/14:2 Tainted: G        W  OE     5.4.0-rc7bhawan+ #480
[  129.374318] Hardware name: System manufacturer System Product Name/PRIME X370-PRO, BIOS 0515 03/30/2017
[  129.374397] Workqueue: events dm_irq_work_func [amdgpu]
[  129.374468] RIP: 0010:dc_link_allocate_mst_payload+0x1f7/0x220 [amdgpu]
[  129.374470] Code: 52 20 e8 1c 63 ad f4 48 8b 5d d0 65 48 33 1c 25 28 00
00 00 b8 01 00 00 00 75 16 48 8d 65 d8 5b 41 5c 41 5d 41 5e 41 5f 5d c3
<0f> 0b e9 fa fe ff ff e8 ed 5b d6 f3 41 0f b6 b6 c4 02 00 00 48 c7
[  129.374471] RSP: 0018:ffff9f9141e7fcc0 EFLAGS: 00010246
[  129.374472] RAX: 0000000000000000 RBX: ffff91ef0762f800 RCX: 0000000000000000
[  129.374473] RDX: 0000000000000005 RSI: ffffffffc0c4a988 RDI: 0000000000000004
[  129.374474] RBP: ffff9f9141e7fd10 R08: 0000000000000005 R09: 0000000000000000
[  129.374475] R10: 0000000000000002 R11: 0000000000000001 R12: ffff91eebd510c00
[  129.374475] R13: ffff91eebd510e58 R14: ffff91ef052c01b8 R15: 0000000000000006
[  129.374476] FS:  0000000000000000(0000) GS:ffff91ef0ef80000(0000) knlGS:0000000000000000
[  129.374477] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  129.374478] CR2: 000055623ea01d50 CR3: 0000000408a8c000 CR4: 00000000003406e0
[  129.374479] Call Trace:
[  129.374550]  dc_link_reallocate_mst_payload+0x12e/0x150 [amdgpu]
[  129.374617]  dc_link_handle_hpd_rx_irq+0x6d4/0x6e0 [amdgpu]
[  129.374693]  handle_hpd_rx_irq+0x77/0x310 [amdgpu]
[  129.374768]  dm_irq_work_func+0x53/0x70 [amdgpu]
[  129.374774]  process_one_work+0x1fd/0x3f0
[  129.374776]  worker_thread+0x255/0x410
[  129.374778]  kthread+0x121/0x140
[  129.374780]  ? process_one_work+0x3f0/0x3f0
[  129.374781]  ? kthread_park+0x90/0x90
[  129.374785]  ret_from_fork+0x22/0x40

[How]
when we disable MST we should clear the cur link settings (lane_count=0 is
good enough). This will cause us to not reallocate payloads earlier than
expected and not throw the warning

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>