drm/i915: Ignore panel type from OpRegion on SKL

Dell XPS 13 9350 apparently doesn't like it when we use the panel type
from OpRegion. The OpRegion panel type (0) tells us to use use low
vswing for eDP, whereas the VBT panel type (2) tells us to use normal
vswing. The problem is that low vswing results in some display flickers.
Since no one seems to know how this stuff is supposed to be handled,
let's just ignore the OpRegion panel type on SKL for now.

v2: Print the panel type correctly in the debug output
Reported-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: drm-intel-fixes@lists.freedesktop.org
References: https://lists.freedesktop.org/archives/intel-gfx/2016-June/098826.html
Fixes: a0562819 ("drm/i915: Get panel_type from OpRegion panel details")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468324837-29237-1-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
(cherry picked from commit bb10d4ec)
[danvet: Fix up cherry-pick conflict with an s/dev_priv/dev/.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_opregion.c

@@ -1038,5 +1038,16 @@ intel_opregion_get_panel_type(struct drm_device *dev)
 		return -ENODEV;
 	}
 
+	/*
+	 * FIXME On Dell XPS 13 9350 the OpRegion panel type (0) gives us
+	 * low vswing for eDP, whereas the VBT panel type (2) gives us normal
+	 * vswing instead. Low vswing results in some display flickers, so
+	 * let's simply ignore the OpRegion panel type on SKL for now.
+	 */
+	if (IS_SKYLAKE(dev)) {
+		DRM_DEBUG_KMS("Ignoring OpRegion panel type (%d)\n", ret - 1);
+		return -ENODEV;
+	}
+
 	return ret - 1;
 }