drm/ast: Fix comment on modeset lock

drm/ast: Fix comment on modeset lock

The ast driver protects the commit tail against concurrent reads
of the display modes by acquiring a lock. The comment is misleading
as the lock is not released in atomic_flush, but at the end of the
commit-tail helper. Rewrite the comment.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 1fe182154984 ("drm/ast: Acquire I/O-register lock in atomic_commit_tail function")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.2+
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250324094520.192974-2-tzimmermann@suse.de