msm_fb : Disable power collapse before VSync to negate wake up latencies.

For a 16ms frame period  the CPU may go into deep power collapse.
Based on the power collapse exit latency for that low power mode,
the frame period gets extended by the latency to wake up the CPU at the
next VSYNC. This results in a user experience glitch. Adding an hrtimer
to wake up the CPU few ms before the Vsync and disabling power collapse
compensates for the extra time spent in waking up the CPU out of deep
sleep. On timer expiry, set wakelock to veto idle power collapse till
the next VSync. The wakelock is released on the next VSYNC occurrence.

Change-Id: I3e013cc54c36d3015d6fafa4b9ccd435cb5532ab
Signed-off-by: Anitha Anand <anithas@codeaurora.org>