msm: mdss: use spin_lock_irqsave while checking the intr status

Use spin_lock_irqsave instead of spin_lock for mdp_lock as the
former would disable the interrupts before acquiring the lock
and saves the previous interrupt state. The states are restored
during spin_unlock_irqrestore. Using raw spin_lock would allow
interrupts and that might lead to deadlock since the mdp_isr_handler
also waits on the same mdp_lock.

Change-Id: If34fe27055b704e9606f772b421662d140fa7ddf
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>