drm/nv50: ignore vbios table's claim to the contrary if EDID says >8bpc

Should fix dim panel issues reported on Dell M6400/M6500.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_display.c

@@ -690,9 +690,21 @@ nv50_display_script_select(struct drm_device *dev, struct dcb_entry *dcbent,
 			   int pxclk)
 {
 	struct drm_nouveau_private *dev_priv = dev->dev_private;
+	struct nouveau_connector *nv_connector = NULL;
+	struct drm_encoder *encoder;
 	struct nvbios *bios = &dev_priv->VBIOS;
 	uint32_t mc, script = 0, or;
 
+	list_for_each_entry(encoder, &dev->mode_config.encoder_list, head) {
+		struct nouveau_encoder *nv_encoder = nouveau_encoder(encoder);
+
+		if (nv_encoder->dcb != dcbent)
+			continue;
+
+		nv_connector = nouveau_encoder_connector_get(nv_encoder);
+		break;
+	}
+
 	or = ffs(dcbent->or) - 1;
 	mc = nv50_display_mode_ctrl(dev, dcbent->type != OUTPUT_ANALOG, or);
 	switch (dcbent->type) {
@@ -711,6 +723,11 @@ nv50_display_script_select(struct drm_device *dev, struct dcb_entry *dcbent,
 			} else
 			if (bios->fp.strapless_is_24bit & 1)
 				script |= 0x0200;
+
+			if (nv_connector && nv_connector->edid &&
+			    (nv_connector->edid->revision >= 4) &&
+			    (nv_connector->edid->input & 0x70) >= 0x20)
+				script |= 0x0200;
 		}
 
 		if (nouveau_uscript_lvds >= 0) {