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: I076a694fcace262a69904b7a3a549dff48345434
Signed-off-by: Pradeep Jilagam <pjilagam@codeaurora.org>
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>