drm/vc4: SCALER_DISPBKGND_AUTOHS is only valid on HVS4

The bit used for SCALER_DISPBKGND_AUTOHS in SCALER_DISPBKGNDX
has been repurposed on HVS5 to configure whether a display can
win back-to-back arbitration wins for the COB.

This is not desirable, therefore only select this bit on HVS4,
and explicitly clear it on HVS5.

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