drm: rcar-du: Enable configurable DPAD0 routing on Gen3

All Gen3 SoCs supported so far have a fixed association between DPAD0
and DU channels, which led to hardcoding that association when writing
the corresponding hardware register. The D3 and E3 will break that
mechanism as DPAD0 can be dynamically connected to either DU0 or DU1.

Make DPAD0 routing dynamic on Gen3. To ensure a valid hardware
configuration when the DU starts without the RGB output enabled, DPAD0
is associated at initialization time to the first DU channel that it can
be connected to. This makes no change on Gen2 as all Gen2 SoCs can
connected DPAD0 to DU0, which is the current implicit default value.

As the DPAD0 source is always 0 when a single source is possible on
Gen2, we can also simplify the Gen2 code in the same function to remove
a conditional check.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>