drm/nv40: add LVDS table quirk for Dell Latitude D620

Should fix:
 https://bugzilla.redhat.com/show_bug.cgi?id=505132
 https://bugzilla.redhat.com/show_bug.cgi?id=543091
 https://bugzilla.redhat.com/show_bug.cgi?id=530425
 https://bugs.edge.launchpad.net/ubuntu/+source/xserver-xorg-video-nouveau/
 +bug/539730
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_bios.c

@@ -3659,6 +3659,21 @@ int nouveau_bios_parse_lvds_table(struct drm_device *dev, int pxclk, bool *dl, b
 		break;
 	}
 
+	/* Dell Latitude D620 reports a too-high value for the dual-link
+	 * transition freq, causing us to program the panel incorrectly.
+	 *
+	 * It doesn't appear the VBIOS actually uses its transition freq
+	 * (90000kHz), instead it uses the "Number of LVDS channels" field
+	 * out of the panel ID structure (http://www.spwg.org/).
+	 *
+	 * For the moment, a quirk will do :)
+	 */
+	if ((dev->pdev->device == 0x01d7) &&
+	    (dev->pdev->subsystem_vendor == 0x1028) &&
+	    (dev->pdev->subsystem_device == 0x01c2)) {
+		bios->fp.duallink_transition_clk = 80000;
+	}
+
 	/* set dual_link flag for EDID case */
 	if (pxclk && (chip_version < 0x25 || chip_version > 0x28))
 		bios->fp.dual_link = (pxclk >= bios->fp.duallink_transition_clk);