drm: renesas: rcar-du: Add drm_panic support for non-vsp

Add support for the drm_panic module for DU variants not using the
VSP-compositor, to display a message on the screen when a kernel panic
occurs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Link: https://patchwork.freedesktop.org/patch/msgid/b633568d2e3f405b21debdd60854fe39780254d6.1716816897.git.geert+renesas@glider.be


Signed-off-by: Maxime Ripard <mripard@kernel.org>