drm/mgag200: Add missing write to index before accessing data register

This is a bug fix for some versions of g200se cards while doing
mode-setting.

Signed-off-by: Christopher Harvey <charvey@matrox.com>
Tested-by: Julia Lemire <jlemire@matrox.com>
Acked-by: Julia Lemire <jlemire@matrox.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@gmail.com>