drm/radeon/kms: fix DVO setup on some r4xx chips

Some r4xx chips have the wrong frev in the
DVOEncoderControl table.  It should always be 1
on r4xx.  Fixes modesetting on DVO on r4xx chips
with the bad frev.

Reported by twied on #radeon.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/atombios_encoders.c

@@ -230,6 +230,10 @@ atombios_dvo_setup(struct drm_encoder *encoder, int action)
 	if (!atom_parse_cmd_header(rdev->mode_info.atom_context, index, &frev, &crev))
 		return;
 
+	/* some R4xx chips have the wrong frev */
+	if (rdev->family <= CHIP_RV410)
+		frev = 1;
+
 	switch (frev) {
 	case 1:
 		switch (crev) {