drm/i915: Also call frontbuffer flip when disabling planes.

drm/i915: Also call frontbuffer flip when disabling planes.

We also need to call the frontbuffer flip to trigger proper
invalidations when disabling planes. Otherwise we will miss
screen updates when disabling sprites or cursor.

On core platforms where HW tracking also works, this issue
is totally masked because HW tracking triggers PSR exit
however on VLV/CHV that has only SW tracking we miss screen
updates when disabling planes.

It was caught with kms_psr_sink_crc sprite_plane_onoff
and cursor_plane_onoff subtests running on VLV/CHV.

This is probably a regression since I can also get this
with the manual test case, but with so many changes on atomic
modeset I couldn't track exactly when this was introduced.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>