drm/msm: Switch to atomic_helper_commit()

Now that all of the msm-specific goo is tucked safely away we can switch
over to using the atomic helper commit directly. \o/

Changes in v2:
- None
Changes in v3:
- Rebased on Archit's private_obj set
Changes in v4:
- None

Cc: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>