drm/msm/sde: add KMS hook to wait for panel transfer complete

Some of the SDE features mandates the current frame to be flushed
out from the SDE HW before programming the next one. This change adds
hook for panel transfer complete in KMS so that atomic commit can wait
on the SDE to complete transfer of the frame to the panel.

Change-Id: I0d873c94ef29be38ed2002c357b18e15bbea698a
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>