drm/mgag200: Reject non-character-cell-aligned mode widths

commit 25161084b1c1b0c29948f6f77266a35f302196b7 upstream.

Turns out 1366x768 does not in fact work on this hardware.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>