drm/mgag200: Rename constant MGAREG_Status to MGAREG_STATUS

Register constants are upper case. Fix MGAREG_Status accordingly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205160142.3588-3-tzimmermann@suse.de