drm/mgag200: Black screen fix for G200e rev 4

- Fixed black screen for some resolutions of G200e rev4
- Fixed testm & testn which had predetermined value.

Reported-by: Jan Beulich <jbeulich@suse.com>

Signed-off-by: Mathieu Larouche <mathieu.larouche@matrox.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>