modetest: add mediatek to module list

There is a mediatek drm kms driver: Add "mediatek" to the static
lists of driver names.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: JB Tsai <jb.tsai@mediatek.com>
Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>