drm/vc4: Set up SCALER_DISPCTRL at boot.

We want the HVS on, obviously, and we also want DSP3 (PV1's source) to
be muxed from HVS channel 2 like we expect in vc4_crtc.c.  The
firmware wasn't setting the DSP3 mux up when both the LCD and HDMI
were disabled.

Signed-off-by: Eric Anholt <eric@anholt.net>