drm/vc4: Drop WARN for HVS FIFOs not being empty

The reset condition for the EMPTY flag in DISPSTATx is 0,
so seeing as we've just reset the pipeline there is no
guarantee that the flag will denote empty if it hasn't been
enabled.

Drop the WARN.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>