drm/vc4: hvs: Ignore atomic_flush if we're disabled

atomic_flush will be called for each CRTC even if they aren't enabled.

The whole code we have there will thus run without a properly affected
channel, which can then result in all sorts of weird behaviour.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20221207-rpi-hvs-crtc-misc-v1-8-1f8e0770798b@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>