drm/ast: Add constants for VGACRCB register bits

Set the bits in VGACRCB with constants. Alo move the rsp code into a
helper function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-2-tzimmermann@suse.de