drm/msm: update mode-2 exit sequence to wait power_evt_12

Mode-2 exit sequence waits for gdsc on only. Ideally, it
should wait for power_evt_12 after before allowing clients
to access any register. Power_evt_12 confirms that
gdsc is powered on and rsc hardware has completed the
mode-2 exit for rsc/pdc. It is OK to wait for only this
event based on hardware programming recommendation.

Change-Id: I3a29e23de4f80cd226da33bc7655f213f2d2401b
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>