drm/mgag200: Pin displayed cursor BO to video memory

The cursor BO has to be pinned to video ram while it's being displayed.
With the current code, the BO might be pinned to system memory instead.
The patch fixes this problem.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Fixes: 94dc57b1 ("drm/mgag200: Rewrite cursor handling")
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190723075425.24028-2-tzimmermann@suse.de