drm/ast: Rework I/O register setup

drm/ast: Rework I/O register setup

There are three different ways of retrieving the I/O-memory ranges
for AST devices: either from PCI BAR 1, from PCI BAR 2 or from PCI
BAR 1 by 'guessing'.

Make the respective code more readable by making each case self-
contained. Also add error checking against the length of the PCI
BARs.

v2:
	* fix I/O range length to 128 bytes
	* fix length test for PCI BAR 2

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116100240.22975-3-tzimmermann@suse.de