drm/ast: Enable and unlock device access early during init

POST and memory management contains code to enable access to the
device's memory spaces. This is too late. Consolidate this code at
the beginning of the device initialization.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com> # AST2600
Link: https://patchwork.freedesktop.org/patch/msgid/20230621130032.3568-8-tzimmermann@suse.de