drm/hisilicon/hibmc: Fix preferred depth and bpp

Set the preferred color depth to 24 bits and the fbdev bpp to 32
bits. This will signal XRGB8888 as default format to clients.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123115348.2521-2-tzimmermann@suse.de