drm/cirrus: Use VGA macro constants to unblank

Set the VGA bit for unblanking with macro constants instead of magic
values. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-18-tzimmermann@suse.de