drm/nv50: preserve an unknown SOR_MODECTRL value for DP encoders

This value interacts with some registers we don't currently know how to
program properly ourselves.  The default of 5 that we were using matches
what the VBIOS on early DP cards do, but later ones use 6, which would
cause nouveau to program an incorrect mode on these chips.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_encoder.h

@@ -47,6 +47,7 @@ struct nouveau_encoder {
 
 	union {
 		struct {
+			int mc_unknown;
 			int dpcd_version;
 			int link_nr;
 			int link_bw;

drivers/gpu/drm/nouveau/nv50_sor.c

@@ -211,7 +211,7 @@ nv50_sor_mode_set(struct drm_encoder *encoder, struct drm_display_mode *mode,
 			mode_ctl = 0x0200;
 		break;
 	case OUTPUT_DP:
-		mode_ctl |= 0x00050000;
+		mode_ctl |= (nv_encoder->dp.mc_unknown << 16);
 		if (nv_encoder->dcb->sorconf.link & 1)
 			mode_ctl |= 0x00000800;
 		else
@@ -274,6 +274,7 @@ static const struct drm_encoder_funcs nv50_sor_encoder_funcs = {
 int
 nv50_sor_create(struct drm_device *dev, struct dcb_entry *entry)
 {
+	struct drm_nouveau_private *dev_priv = dev->dev_private;
 	struct nouveau_encoder *nv_encoder = NULL;
 	struct drm_encoder *encoder;
 	bool dum;
@@ -319,5 +320,24 @@ nv50_sor_create(struct drm_device *dev, struct dcb_entry *entry)
 	encoder->possible_crtcs = entry->heads;
 	encoder->possible_clones = 0;
 
+	if (nv_encoder->dcb->type == OUTPUT_DP) {
+		uint32_t mc, or = nv_encoder->or;
+
+		if (dev_priv->chipset < 0x90 ||
+		    dev_priv->chipset == 0x92 || dev_priv->chipset == 0xa0)
+			mc = nv_rd32(dev, NV50_PDISPLAY_SOR_MODE_CTRL_C(or));
+		else
+			mc = nv_rd32(dev, NV90_PDISPLAY_SOR_MODE_CTRL_C(or));
+
+		switch ((mc & 0x00000f00) >> 8) {
+		case 8:
+		case 9:
+			nv_encoder->dp.mc_unknown = (mc & 0x000f0000) >> 16;
+			break;
+		default:
+			break;
+		}
+	}
+
 	return 0;
 }