drm/msm/sde: toggle vblank using enable state in display thread

Add enabled flag to the CRTC to be able to track the enabled or
disabled status in the display thread instead of the atomic
state. The atomic state is swapped prior to the display thread
dispatch. This is before the display thread executes the enable
or disable operation, so the state is not properly reflected for
a running VSYNC work item.

Also, simplify the CRTC VBLANK function structure to use a
single common function call to update the CRTC's VBLANK
registration with the encoder to avoid extra refs and unrefs.

Change-Id: I9b473becec15427b03b5ebf0333e10e4911dfd9b
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>