drm/renesas/rcar-du: Run DRM default client setup

drm/renesas/rcar-du: Run DRM default client setup

Call drm_client_setup() to run the kernel's default client setup
for DRM. Set fbdev_probe in struct drm_driver, so that the client
setup can start the common fbdev client.

The rcar-du driver specifies a preferred color mode of 32. As this
is the default if no format has been given, leave it out entirely.

v5:
- select DRM_CLIENT_SELECTION

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240924071734.98201-34-tzimmermann@suse.de