drm/mgag200: Fix logic in mgag200_bo_pin() (v2)

Add missing 'return 0;'.

v2: Simplified patch as suggested by Dave Airlie <airlied@redhat.com>

Signed-off-by: Egbert Eich <eich@suse.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>