drm: Add kerneldoc description for "link"-member in struct drm_panel

Add kerneldoc description for "struct device_link *link"-member in
struct drm_panel.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1534352611-19074-1-git-send-email-jsarha@ti.com

include/drm/drm_panel.h

@@ -82,6 +82,7 @@ struct drm_panel_funcs {
  * @drm: DRM device owning the panel
  * @connector: DRM connector that the panel is attached to
  * @dev: parent device of the panel
+ * @link: link from panel device (supplier) to DRM device (consumer)
  * @funcs: operations that can be performed on the panel
  * @list: panel entry in registry
  */