drm/solomon: Set preferred color depth and bpp to the correct values

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/20221123115348.2521-7-tzimmermann@suse.de