drm/of: Fix kerneldoc

I noticed a link that didn't work ...

Fixes: 1f2db303 ("drm: of: introduce drm_of_find_panel_or_bridge")
Cc: Rob Herring <robh@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111164048.29067-2-daniel.vetter@ffwll.ch