drm/mgag200: Acquire I/O-register lock in atomic_commit_tail function

Hold I/O-register lock in atomic_commit_tail to protect all pipeline
updates at once. Protects against concurrent I/O access in get-modes
helper.

Complex modesetting operations involve mode changes, plane updates and
possibly BMC updates. Make all this atomic wrt to reading display modes
via EDID. It's not so much an issue with simple-KMS helpers, but will
become necessary for using regular atomic helpers.

v4:
	* remove empty line

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728124103.30159-5-tzimmermann@suse.de