drm/mediatek: Handle component type MTK_DISP_OVL_2L correctly

The larb device remains NULL if the type is MTK_DISP_OVL_2L.
A kernel panic is raised when a crtc uses mtk_smi_larb_get or
mtk_smi_larb_put.

Fixes: b17bdd0d

 ("drm/mediatek: add component OVL_2L0")

Signed-off-by: Phong LE <ple@baylibre.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>