drm: rz-du: Support panels connected directly to the DPAD output

drm: rz-du: Support panels connected directly to the DPAD output

This patch is based on the commit 73eb5476df72 ("drm: rcar-du: Support
panels connected directly to the DPAD outputs").

The RZ DU driver assumes that a bridge is always connected to the DU
output. This is valid for the HDMI output, but the DPAD output can be
connected directly to a panel, in which case no bridge is available.

To support this use case, detect whether the entities connected to the DU
DPAD output is encoders or panels based on the number of ports of their DT
node, and retrieve the corresponding type of DRM objects. For panels,
additionally create panel bridge instances.

Signed-off-by: hienhuynh <hien.huynh.px@renesas.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Tested-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Link: https://lore.kernel.org/r/20250508095042.25164-1-biju.das.jz@bp.renesas.com