drm/msm/dpu: squash power handle event types

DPU power handler maintained PRE/POST versions of power
ENABLE/DISABLE events to accommodate tasks which need be
handled before/after data bus voting. But since the bus voting
API's are deprecated and removed from the driver, squash
the events and their clients respective event handlers
to handle only ENABLE/DISABLE events.

changes in v5:
	- introduced in the series

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
[seanpaul converted #defines to BIT(x) in dpu_power_handle.h]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>