drm/mgag200: Set SCROFF in primary-plane code

The SCROFF bit controls reading the primary plane's scanout buffer
from video memory. Set it from primary-plane code, instead of CRTC
code.

v3:
	* only flip SCROFF when enabling/disabling the plane (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728124103.30159-9-tzimmermann@suse.de