drm/fb-helper: Introduce drm_fb_helper_unprepare()

Move the fb-helper clean-up code into drm_fb_helper_unprepare(). No
functional changes.

v2:
	* declare as static inline (kernel test robot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125200415.14123-4-tzimmermann@suse.de