drm/msm/sde: ensure rsc in vid state at writeback null commit

In few scenarios during doze mode transitions and when there is
change in writeback output buffer format to YUV, rsc is in idle or
clock state where NULL flush is triggered in writeback path. Due to
this WB CTL path is hung and next consecutive commits are leading to
timeouts. The change made ensures rsc is in vid mode in such scenarios.

Change-Id: If093c3c9d2c5546e902e5dddd8ae6b9bea51a616
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>