drm/radeon/kms: add R4XX mc register access helper.

Atombios will use the mc register access helper and R4XX hw have a
bigger mc range than R3XX so add R4XX specific mc register access
helper.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>