drm/ast: 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 modesetting against concurrent EDID reads.

Complex modesetting operations involve mode changes and plane updates.
These steps used to be protected individually against concurrent I/O.
Make all this atomic wrt to reading display modes via EDID. The EDID
code in the connector's get_modes helper already acquires the necessary
lock.

A similar issue was fixed in commit 2d70b9a1 ("drm/mgag200: Acquire
I/O-register lock in atomic_commit_tail function") for mgag200.

v2:
	* fix typo in commit message (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221013112923.769-2-tzimmermann@suse.de