msm: mdss: Ensure event timer is added only once for MDP irq

MDP driver calls add_event_timer multiple times, resulting
in creating multiple event timers. Due to this irqbalancer
changes MDP irq's  affinity to different cpu. Ensure that
add_event_timer is added only once for MDP irq.

Change-Id: If0425ef5a3b3ce56c40da52ff3ced6658f05734a
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>