drm/debugfs: add an "edid_override" file per connector

Add a file to debugfs for each connector to allow the EDID to be
overridden.

v2: Copy ubuf before accessing it and reject invalid length data. (David
    Herrmann)
    Ensure override_edid is reset when a new EDID value is written.
    (David Herrmann)
    Fix the debugfs file permissions. (David Herrmann)

Signed-off-by: Thomas Wood <thomas.wood@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
index d22676b..db7d250 100644
--- a/drivers/gpu/drm/drm_probe_helper.c
+++ b/drivers/gpu/drm/drm_probe_helper.c
@@ -130,7 +130,14 @@
 	count = drm_load_edid_firmware(connector);
 	if (count == 0)
 #endif
-		count = (*connector_funcs->get_modes)(connector);
+	{
+		if (connector->override_edid) {
+			struct edid *edid = (struct edid *) connector->edid_blob_ptr->data;
+
+			count = drm_add_edid_modes(connector, edid);
+		} else
+			count = (*connector_funcs->get_modes)(connector);
+	}
 
 	if (count == 0 && connector->status == connector_status_connected)
 		count = drm_add_modes_noedid(connector, 1024, 768);