drm/mediatek: Use correct aliases name for ovl

Aliases property name must include only lowercase and '-', so fix this
in the driver, so we're not tempted to do "ovl_2l0 = &ovl_2l0" in the
device-tree instead of the right one which is "ovl-2l0 = &ovl_2l0".

Fixes: b17bdd0d ("drm/mediatek: add component OVL_2L0")
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>