plane->state->fb and plane->fb should always reference the same FB so
that atomic and legacy codepaths have the same view of display state.
However, there are some places in kernel code that directly set
plane->fb and neglect to update plane->state->fb. If we never do a
successful update through the atomic pipeline, the RmFB cleanup code
will look at the plane->state->fb pointer, which has never actually
been set to a legitimate value, and try to clean it up, leading to
BUG's.

Add a quick helper function to synchronize plane->state->fb with
plane->fb and call it everywhere the driver tries to manually set
plane->fb outside of the atomic pipeline. In this function, use
drm_atomic_set_fb_for_plane instead of writing plane->state->fb
directly to keep the reference count right.

This is modified from Matt Roper's patch to drm-intel-nightly with
commit id

commit afd65eb4cc0578a9c07d621acdb8a570e2782bf7
Author: Matt Roper <matthew.d.roper@intel.com>
Date:   Tue Feb 3 13:10:04 2015 -0800

    drm/i915: Ensure plane->state->fb stays in sync with plane->fb

However this bug exists in mainline kernel too, so I created this to fix
it in mainline kernel.

A minor change is to use drm_atomic_set_fb_for_plane instead of update
reference count manually.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88909
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=93711
Signed-off-by: Xi Ruoyao <xry111@outlook.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
[Jani: included the patch notes in the commit message]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>