drm/msm/dsi-staging: remove unnecesary locks from commit path

Frame drops are observed when crtc_commit thread tries to hold panel
lock for updating the frame, while backlight service thread is
pushing the backlight commands holding panel_lock. Remove holding
panel lock from commit thread.

CRs-Fixed: 2179313
Change-Id: I7d6e5044db570102b7c2c7edb1c5c4b24847ed3f
Signed-off-by: Vara Reddy <varar@codeaurora.org>