drm/logicvc: Fix preferred fbdev cpp

Logicvc can have different values for the preferred color depth. Set
the fbdev bpp value depending on the runtime value.

v2:
	* remove unused color depth of 15 from switch (Javier)

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/20221123115348.2521-3-tzimmermann@suse.de