disp: msm: sde: add vblank mutex lock during irq unregister

Currently, during ctl reset in video mode, irq register or
unregister might result in race condition with vblank
enable/disable calls on event thread resulting in enable
cnt mismatch. This change adds mutex locks to avoid race
conditions in such cases.

Change-Id: I45aef19864475ac1b02dd8e84810eee233fc60ea
Signed-off-by: Yashwanth <yvulapu@codeaurora.org>