drm/vc4: kms: Assign a FIFO to enabled CRTCs instead of active

The HVS has three FIFOs that can be assigned to a number of PixelValves
through a mux.

However, changing that FIFO requires that we disable and then enable the
pixelvalve, so we want to assign FIFOs to all the enabled CRTCs, and not
just the active ones.

Fixes: 87ebcd42

 ("drm/vc4: crtc: Assign output to channel automatically")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200918145918.101068-1-maxime@cerno.tech