drm/i915/dp: force eDP lane count to max available lanes on BDW

There are certain BDW high res eDP machines that regressed due to

commit 38aecea0ccbb909d635619cba22f1891e589b434
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Mar 3 11:18:10 2014 +0100

    drm/i915: reverse dp link param selection, prefer fast over wide again

The commit lead to 2 lanes at 5.4 Gbps being used instead of 4 lanes at
2.7 Gbps on the affected machines. Link training succeeded for both, but
the screen remained blank with the former config. Further investigation
showed that 4 lanes at 5.4 Gbps worked also.

The root cause for the blank screen using 2 lanes remains unknown, but
apparently the driver for a certain other operating system by default
uses the max available lanes. Follow suit on Broadwell eDP, for at least
until we figure out what is going on.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76711
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Tested-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_dp.c

@@ -810,7 +810,12 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 			bpp = dev_priv->vbt.edp_bpp;
 		}
 
-		if (dev_priv->vbt.edp_lanes) {
+		if (IS_BROADWELL(dev)) {
+			/* Yes, it's an ugly hack. */
+			min_lane_count = max_lane_count;
+			DRM_DEBUG_KMS("forcing lane count to max (%u) on BDW\n",
+				      min_lane_count);
+		} else if (dev_priv->vbt.edp_lanes) {
 			min_lane_count = min(dev_priv->vbt.edp_lanes,
 					     max_lane_count);
 			DRM_DEBUG_KMS("using min %u lanes per VBT\n",