msm: mdss: add debug traces to measure blank time

Add systrace events to log the time to process a blank
request; this helps to profile the time that display
driver takes during the power transitions.

Change-Id: I0081c3825a4776561db248123d701c25101e9245
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>