drm/msm/sde: remove redundant vblank request and notification

Vblank is requested twice in sde, once in sde_wait_for_commit_done
and another one in request_pending for each commit cycle. Only one
is required; hence, remove request_pending and associated logics.

Change-Id: I1ca69921abd5f7ab74b8f8900aee52e22efbe350
Signed-off-by: Alan Kwong <akwong@codeaurora.org>