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