drm/msm/sde: move UBWC init into SDE_POWER_EVENT_POST_ENABLE event

Move the UBWC initialization into SDE_POWER_EVENT_POST_ENABLE event,
to trigger when V4L2 driver votes for DPU shared resources.
This is required for standalone V4L2 offline rotation with UBWC formats
which previously failed if there was no prior commit to the display
due to UBWC HW block not being initialized.

Change-Id: Ifc2d1509447f3f95f04630b79f266882a531345e
Signed-off-by: Alexander Beykun <abeykun@codeaurora.org>