drm/nv50/dp: fix hack to work for macbooks booted via EFI

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_sor.c

@@ -318,6 +318,8 @@ nv50_sor_create(struct drm_connector *connector, struct dcb_entry *entry)
 		uint32_t tmp;
 
 		tmp = nv_rd32(dev, 0x61c700 + (or * 0x800));
+		if (!tmp)
+			tmp = nv_rd32(dev, 0x610798 + (or * 8));
 
 		switch ((tmp & 0x00000f00) >> 8) {
 		case 8: