drm/msm/sde: increment pending kickoff count after trigger start

Increment the pending kickoff counter after the CTL start is
triggered.  This resolves a race condition when the vblank
IRQ occurs after this counter has been incremented but before
the new flush settings have been written to the HW causing
frame buffers to be unmapped while HW is still accessing them.

Change-Id: Idb5969c4349d4c036ca4c1739c920b6bebc97dd8
Signed-off-by: Steve Cohen <cohens@codeaurora.org>