drm/mediatek: check for memory allocation failure

If 'devm_kmalloc_array' returns NULL, we should return -ENOMEM as already
done a few lines above instead of deferencing a NULL pointer a few lines
below.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>