drm/msm: use wait for vblank instead of wait for commit done

For programming seamless transitions we need to update DSI and
SDE timing programming together just after a vblank. We should
use the standard DRM wait for vblank helper function to do
the waiting rather than the overloaded wait for commit done
which has greater callflow implications.

CRs-Fixed: 2005394
Change-Id: I78a96e314c3a7f6f383de45aebac981247d88449
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>