msm_fb: display: remove while loop in mdp ISR

As mdp interrupt is enabled on-demand, there is no need to have
a while loop to handle interrupts at random time. There is possibility
that mdp clock is turned off at first loop and ISR handler try to
read mdp status register again at second loop which cause system to reset.

Change-Id: I3716638974d8e24eda4a5146421d026b0ca85b4c
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>