mdss: rotator: increase rotator commit count before scheduling work

The commit count keeps track of pending commits in rotator to create
proper sync fences. If a rotator work is currently pending when a new
rotator buffer is being queued, the release fence returned could still
be with regards to previous frame. This can cause corruption by since
the new fence timeline wouldn't account for the pending rotator work
items. Fix this by increasing commit count as soon as work is scheduled.

Change-Id: I1cb6af54a4c83a265139f2243b70726eea989512
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>