drm/vc4: Ignore HVS unless initialised

An upstream commit to report HVS underruns causes VC4 in firmware KMS
mode to cross into the HVS side, where it crashes due to a NULL hvs
pointer.

Make the underrun masking conditional on the hvs pointer being
initialised.

Fixes: 531a1b62 ("drm/vc4: Report HVS underrun errors")

Signed-off-by: Phil Elwell <phil@raspberrypi.org>