drm/debugfs: add a "force" file per connector

Add a file to debugfs for each connector to enable modification of the
"force" connector attribute. This allows connectors to be enabled or
disabled for testing and debugging purposes.

v2: Add stricter value checking and clean up debugfs_entry if file
    creation fails in drm_debugfs_connector_add. (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/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 5512c99..d4b0a6a 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -545,6 +545,8 @@
 	int audio_latency[2];
 	int null_edid_counter; /* needed to workaround some HW bugs where we get all 0s */
 	unsigned bad_edid_counter;
+
+	struct dentry *debugfs_entry;
 };
 
 /**