drm/atomic-helper: Add atomic_enable plane-helper callback

Add atomic_enable to struct drm_plane_helper_funcs. It enables a
plane independently from updating the plane's content. As such, it is
the inverse of the atomic_disable plane helper. Useful for hardware
where plane enable state is independent from plane content.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230209154107.30680-2-tzimmermann@suse.de