msm: mdss: implement display thread to handle commits

Refactor sync point implementation by using a commit thread to handle
all commits for better performance. Make use of commit count to
synchronize the release fences with the right point in the timeline.
Keep a separate commits pending variable to be able to queue multiple
commits as well.

Change-Id: I1303c49f2c56eb5b9a34c162a1896670d307a674
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>