drm/ast: Rework definition of I/O read and write helpers

Ast defines a number of I/O helpers for accessing hardware. Only 4 of
the many generated functions are actually used. Replace the respective
generator macros with those 4 functions. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230221155745.27484-3-tzimmermann@suse.de