drm/msm/sde: move updating pending kickoff count before start

Move update of the pending kickoff count to before trigger start.
Avoid jank caused by CTL_START triggering before the counter is
updated in cases when the interrupt comes immediately after trigger.

Change-Id: I388b8d724fae77bb71838d267f2510ac440fbcb7
Signed-off-by: Steve Cohen <cohens@codeaurora.org>