drm/radeon/kms: fix washed out image on legacy tv dac

bad cast was overwriting the tvdac adj values
Fixes fdo bug 27478

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_connectors.c

@@ -315,7 +315,7 @@ int radeon_connector_set_property(struct drm_connector *connector, struct drm_pr
 		radeon_encoder = to_radeon_encoder(encoder);
 		if (!radeon_encoder->enc_priv)
 			return 0;
-		if (rdev->is_atom_bios) {
+		if (ASIC_IS_AVIVO(rdev) || radeon_r4xx_atom) {
 			struct radeon_encoder_atom_dac *dac_int;
 			dac_int = radeon_encoder->enc_priv;
 			dac_int->tv_std = val;