drm: Loongson-3 doesn't fully support wc memory

[ Upstream commit 221004c66a58949a0f25c937a6789c0839feb530 ]

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Cc: stable@vger.kernel.org
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>