Not all writeback connector implementations might want to commit things
from the connector driver. Some, like the malidp driver, commit things
from their main commit_tail() function, and would rather not have to
implement a dummy hook for drm_connector_helper_funcs.atomic_commit().

Make this function optional and reflect this fact in the doc.
Signed-off-by: NBoris Brezillon <boris.brezillon@bootlin.com>
Acked-by: NLiviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703075022.15138-4-boris.brezillon@bootlin.com