drm/msm/sde: handle VSYNC_IN_EN during crtc power events

Currently VSYNC_IN_EN is closely tied with frame updates,
it is enabled during the kickoff and disabled as part of
idle power collapse sequence. This will impact vblank
request without a frame update during idle power collapse,
as the RD_PTR_IRQ will be triggered only after the wraparound.
Disable VSYNC_IN_EN during PRE_DISABLE power event, to ensure
no vsync is triggered when RSC enters mode-2. Enable it during
POST_ENABLE power event as part of encoder restore and during
virtual encoder enable to support bootup/resume cases.

Change-Id: Iea6d6e9617a9db2c2518695ff5a428a97f887a2d
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>