drm/vc4: Ensure vc4_hdmi doesn't use 2711 HPD registers on Pi0-3

The existing logic was flawed in that it could try reading the
2711 specific registers for HPD on a CM1/3 where the HPD GPIO
hadn't been defined in DT.

Ensure we don't do the 2711 register read on invalid hardware,
and then

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