drm/drm_debugfs_crc.c: Document that .verify_crc_source vfunc is required for enabling CRC support.

drm_debugfs_crtc_crc_add() function checks that both .set_crc_source and
.verify_crc_source hooks are provided before enabling debugfs support for
reading per-frame CRC data. Make that explicit in the documentation.

Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703150330.21992-1-Liviu.Dudau@arm.com

drivers/gpu/drm/drm_debugfs_crc.c

@@ -66,9 +66,9 @@
  * the reported CRCs of frames that should have the same contents.
  *
  * On the driver side the implementation effort is minimal, drivers only need to
- * implement &drm_crtc_funcs.set_crc_source. The debugfs files are automatically
- * set up if that vfunc is set. CRC samples need to be captured in the driver by
- * calling drm_crtc_add_crc_entry().
+ * implement &drm_crtc_funcs.set_crc_source and &drm_crtc_funcs.verify_crc_source.
+ * The debugfs files are automatically set up if those vfuncs are set. CRC samples
+ * need to be captured in the driver by calling drm_crtc_add_crc_entry().
  */
 
 static int crc_control_show(struct seq_file *m, void *data)