drm/radeon/dp: add back special handling for NUTMEG

BugLink: http://bugs.launchpad.net/bugs/1600092

When I fixed the dp rate selection in:
092c96a8
drm/radeon: fix dp link rate selection (v2)
I accidently dropped the special handling for NUTMEG
DP bridge chips.  They require a fixed link rate.
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Tested-by: Ken Moffat <zarniwhoop@ntlworld.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
(cherry picked from commit c8213a63)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/gpu/drm/radeon/atombios_dp.c

@@ -315,15 +315,27 @@ int radeon_dp_get_dp_link_config(struct drm_connector *connector,
 	unsigned max_lane_num = drm_dp_max_lane_count(dpcd);
 	unsigned lane_num, i, max_pix_clock;
 
-	for (lane_num = 1; lane_num <= max_lane_num; lane_num <<= 1) {
-		for (i = 0; i < ARRAY_SIZE(link_rates) && link_rates[i] <= max_link_rate; i++) {
-			max_pix_clock = (lane_num * link_rates[i] * 8) / bpp;
+	if (radeon_connector_encoder_get_dp_bridge_encoder_id(connector) ==
+	    ENCODER_OBJECT_ID_NUTMEG) {
+		for (lane_num = 1; lane_num <= max_lane_num; lane_num <<= 1) {
+			max_pix_clock = (lane_num * 270000 * 8) / bpp;
 			if (max_pix_clock >= pix_clock) {
 				*dp_lanes = lane_num;
-				*dp_rate = link_rates[i];
+				*dp_rate = 270000;
 				return 0;
 			}
 		}
+	} else {
+		for (lane_num = 1; lane_num <= max_lane_num; lane_num <<= 1) {
+			for (i = 0; i < ARRAY_SIZE(link_rates) && link_rates[i] <= max_link_rate; i++) {
+				max_pix_clock = (lane_num * link_rates[i] * 8) / bpp;
+				if (max_pix_clock >= pix_clock) {
+					*dp_lanes = lane_num;
+					*dp_rate = link_rates[i];
+					return 0;
+				}
+			}
+		}
 	}
 
 	return -EINVAL;