drm/panel: Small documentation polish

Need to make sure people can find the panel-bridge to avoid typing too
much.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111164048.29067-3-daniel.vetter@ffwll.ch