drm/ast: Implement register helpers in ast_drv.h

There are already a number of register I/O functions in ast_drv.h.
For consistency, move the remaining functions there as well. No
functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com> # AST2600
Link: https://patchwork.freedesktop.org/patch/msgid/20230621130032.3568-4-tzimmermann@suse.de