drm/msm: Refactor complete_commit() to look more the helpers

Factor out the commit_tail() portions of complete_commit() into a
separate function to facilitate moving to the atomic helpers in future
patches.

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

Cc: Jeykumar Sankaran <jsanka@codeaurora.org>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>