drm/gem: Warn on trying to use a non-existing framebuffer plane

Warn if callers of drm_gem_fb_get_obj() try to use a GEM buffer for
a non-existing or unset plane.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220517113327.26919-6-tzimmermann@suse.de