drm/msm: Add .commit() callback to msm_kms functions

Called right before wait_for_commit_done() to perform kickoff for
active crtcs.

Changes in v3:
- None

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
[seanpaul split this out of the megapatch]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>