drm/msm: dispatch sync commit to display thread

Current implementation dispatches sync commit in the
calling thread context instead of the display thread
context. This will cause serialization issues between
idle pc, atomic commit, and atomic check operations.
Dispatch sync commit to the corresponding display
thread to serialize processing properly.

Change-Id: I710e0daecfa659a00948092ff15dfc699f5f9344
Signed-off-by: Alan Kwong <akwong@codeaurora.org>