msm_fb: display: add spinlock to synchronize mdp irq enable/disable

To improve performance, UI push thread release mutex before blocked at
waiting for vsync to allow Video thread to be updated at same vsycn.
This also cause contention to enable/disable mdp irq.
Add spinlock to synchronize both threads and irq context.

Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>