drm/msm/sde: add cpu event timer triggered at VSYNC boundary

Enable cpu event timer on VSYNC boundary which supports video and
command mode panel. Cpu timer event is calculated at the time of
kickoff, based on the delta between the current vertical line counter
and the panel vtotal. The timer event is used for waking up CPU on
VSYNC boundary in order to reduce latency for VSYNC/rd_ptr interrupt
handling.

Change-Id: I5c99b746213a527475ecc44a82b1df6c44da0b67
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>