msm_fb: display: use mutex instead of spin lock to avoid deadlock

There has deadlock between mdp4_isr() and disable_irq() called by
LCDC thread. Disable_irq() will wait for mdp4_isr() to complete
before return. Meanwhile mdp4_isr() will try to acquire mdp_spin_lock
which had been acquired by lcdc thread before call disable_irq().
This patch replace spin lock with mutex to avoid deadlock from happening.

CRs-fixed: 301276
Signed-off-by: kuogee hsieh <khsieh@codeaurora.org>