msm_fb : Add timeout while waiting for vsync event

Avoid eternal wait for vsync event when client tries to
read the corresponding sysfs entry. This will avoid hangs when
client tries to read in corner cases where the Timing
Generator or the clock or IRQ gets disabled.

CRs-fixed: 406752
Change-Id: I773687220b868823a9e4bfdae74361ee6487c5c6
Signed-off-by: Pradeep Jilagam <pjilagam@codeaurora.org>
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>