drm/ast: Turn ioregs_lock to modeset_lock

The lock for the I/O registers is only relevant during mode-setting
operations. It protects the registers from concurrent access from
reading EDID information.

Reduce lock coverage to mode setting, rename the lock and move it
entirely into the mode-setting code. No functional changes, as the
I/O lock was never used for anything else than mode setting.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116100240.22975-2-tzimmermann@suse.de