drm/msm: Use customized function to wait for atomic commit done

MDP FLUSH registers could indicate if the previous flush updates
has taken effect at vsync boundary. Making use of this H/W feature
can catch the vsync that happened between CRTC atomic_flush and
*_wait_for_vblanks, to avoid unnecessary wait.

This change allows kms CRTCs to use their own *_wait_for_commit_done
functions to wait for FLUSH register cleared at vsync, before commit
completion.

Signed-off-by: Hai Li <hali@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>