drm/dp: Clarify DP AUX registration time

drm/dp: Clarify DP AUX registration time

The docs we had for drm_dp_aux_init() and drm_dp_aux_register() were mostly
correct, except for the fact that they made the assumption that all AUX
devices were grandchildren of their respective DRM devices. This is the
case for most normal GPUs, but is almost never the case with SoCs and
display bridges. So, let's fix this documentation to clarify when the right
time to use drm_dp_aux_init() or drm_dp_aux_register() is.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210423184309.207645-5-lyude@redhat.com
Reviewed-by: Dave Airlie <airlied@redhat.com>