drm/gma500/psb: Actually use VBT mode when it is found

With LVDS we were incorrectly picking the pre-programmed mode instead of
the prefered mode provided by VBT. Make sure we pick the VBT mode if
one is provided. It is likely that the mode read-out code is still wrong
but this patch fixes the immediate problem on most machines.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=78562
Cc: <stable@vger.kernel.org>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170418114332.12183-1-patrik.r.jakobsson@gmail.com

drivers/gpu/drm/gma500/psb_intel_lvds.c

@@ -759,20 +759,23 @@ void psb_intel_lvds_init(struct drm_device *dev,
 		if (scan->type & DRM_MODE_TYPE_PREFERRED) {
 			mode_dev->panel_fixed_mode =
 			    drm_mode_duplicate(dev, scan);
+			DRM_DEBUG_KMS("Using mode from DDC\n");
 			goto out;	/* FIXME: check for quirks */
 		}
 	}
 
 	/* Failed to get EDID, what about VBT? do we need this? */
-	if (mode_dev->vbt_mode)
+	if (dev_priv->lfp_lvds_vbt_mode) {
 		mode_dev->panel_fixed_mode =
-		    drm_mode_duplicate(dev, mode_dev->vbt_mode);
+			drm_mode_duplicate(dev, dev_priv->lfp_lvds_vbt_mode);
 
-	if (!mode_dev->panel_fixed_mode)
-		if (dev_priv->lfp_lvds_vbt_mode)
-			mode_dev->panel_fixed_mode =
-				drm_mode_duplicate(dev,
-					dev_priv->lfp_lvds_vbt_mode);
+		if (mode_dev->panel_fixed_mode) {
+			mode_dev->panel_fixed_mode->type |=
+				DRM_MODE_TYPE_PREFERRED;
+			DRM_DEBUG_KMS("Using mode from VBT\n");
+			goto out;
+		}
+	}
 
 	/*
 	 * If we didn't get EDID, try checking if the panel is already turned
@@ -789,6 +792,7 @@ void psb_intel_lvds_init(struct drm_device *dev,
 		if (mode_dev->panel_fixed_mode) {
 			mode_dev->panel_fixed_mode->type |=
 			    DRM_MODE_TYPE_PREFERRED;
+			DRM_DEBUG_KMS("Using pre-programmed mode\n");
 			goto out;	/* FIXME: check for quirks */
 		}
 	}