drm/amdgpu/display: fix aux registration (v2)

We were registering the aux device in the MST late_register
rather than the regular one.

v2: handle eDP as well

Fixes: 405a1f90 ("drm/amdgpu/display: split dp connector registration (v4)")
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1100


Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>