drm/msm/sde: turn on timing engine for only master intf

Current SDE driver turns on timing engine for master
and slave intfs for split display. It does not cause any
issue but it is advisable to turn on timing engine only
on master intf instead of both intfs. SW should also wait
for extra vsync on master intf if timing engine status
bit is high after "disable + vsync wait". HW takes extra
vsync wait to turn off timing engine if timing engine
disable request is during prefetch area with programmable
fetch is enabled.

Change-Id: Ice4949a3ecb0e1a76555497a104d50228ef13381
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>