drm/dp: Export symbol / kerneldoc fixes for DP AUX bus

While working on the DP AUX bus code I found a few small things that
should be fixed. Namely the non-devm version of
of_dp_aux_populate_ep_devices() was missing an export. There was also
an extra blank line in a kerneldoc and a kerneldoc that incorrectly
documented a return value. Fix these.

Fixes: aeb33699 ("drm: Introduce the DP AUX bus")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220510122726.v3.1.Ia91f4849adfc5eb9da1eb37ba79aa65fb3c95a0f@changeid