drm/ast: Add I/O helpers without ast device

Implement I/O access in helpers that do not use an ast device
instance, but the raw pointer to the I/O memory. We'll later need
these helpers to detect the device type before allocating the ast
device instance.

v3:
	* fix typo in commit message (Sui)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116100240.22975-5-tzimmermann@suse.de