drm/msm/sde: single kickoff to flush hardware

Existing callflow has multiple points of flush to hardware.
This complicates the callflows, and makes satisfying command mode
and writeback cases difficult since they require triggers based on
event complete irqs. It is important to avoid programming the next
frame's data into hw before the previous frame has completed. For these
reasons we need to buffer flushing of the ctl path, and trigger it
at kickoff time.

This change uses the kickoff callback from the msm layer to consolidate
simple page flip commits and enable/disable commit callflows into a
common kickoff flow.

Change-Id: I565afcf67969da3dbe6e358b5daeffc713b2027c
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>