msm: mdss: disable irq to enforce mutex within dsi_event_thread

Mdp spin lock is used in dsi_event_thread and in irq context.
Disable irq while holding spinlock to avoid context switching
into irq context and cause spinlock recursion

CRs-fixed: 652850
Change-Id: I4bd8cadb1f5d83acc7e866344b7585e7944e08ac
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>