Writeback connectors do not produce any on-screen output and require
special care for use. Such connectors are hidden from enumeration in
DRM resources by default, but they are still picked-up by fbdev.
This makes rather little sense since fbdev is not really adapted for
dealing with writeback.

Moreover, this is also a source of issues when userspace disables the
CRTC (and associated plane) without detaching the CRTC from the
connector (which is hidden by default). In this case, the connector is
still using the CRTC, leading to am "enabled/connectors mismatch" and
eventually the failure of the associated atomic commit. This situation
happens with VC4 testing under IGT GPU Tools.

Filter out writeback connectors in the fbdev helper to solve this.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Maxime Ripard <maxime.ripard@bootlin.com>
Fixes: 935774cd71fe ("drm: Add writeback connector type")
Cc: <stable@vger.kernel.org> # v4.19+
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181115163248.21168-1-paul.kocialkowski@bootlin.com