drm/msm/sde: reset ctl path registers when wb encoder disabled

In MDP 5.0, when the wb encoder is disabled, the
control path must be reset. This change uses the
same reset logic already used in the video
encoder. The existing reset logic is modified to
take care of writeback. The reset logic for each
phys encoder is moved to a single helper function
where possible. If there is a cdm block attached
to the physical encoder that is being disabled,
it is now unbinded from the pingpong block. The
control flush bits are now set using the ops APIs
instead of hardcoding them in the post disable
function.

Change-Id: I5cc8668d10b7192d4822964ec2feadcf1b50d925
Signed-off-by: Fuad Hossain <fhossain@codeaurora.org>