drm: rcar-du: Disable unused DPAD outputs

drm: rcar-du: Disable unused DPAD outputs

DU channels are routed to DPAD outputs in an SoC-dependent way. The
routing can be fixed (e.g. DU3 to DPAD0 on H3) or configurable (e.g. DU0
or DU1 to DPAD0 on D3/E3). The hardware offers no option to disconnect
DPAD outputs, which are thus always driven by a DU channel.

On SoCs that have less DU channels than DU outputs, such as D3 and E3,
the DPAD output is always driven when all channels are in use by other
outputs (such as the internal LVDS and HDMI encoders). This creates an
unwanted clone on the DPAD output.

However, the parallel output of the DU channels routed to DPAD can be
set to fixed levels in the DU channels themselves through the DOFLR
group register. Use this to turn the DPAD on or off by driving fixed
signals at the output of any DU channel not routed to a DPAD output.
This doesn't affect the DU output signals going to other outputs.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>