drm/amdgpu: warn if dp aux is still attached on free

If this happens (and it recently did), we free a structure while part of
it is still in use, which results in non-obvious crashes. The way it's
detached is not trivial (DRM core has to call the connector .destroy
callback and things must be torn down in the right order), so better
detect it and warn early.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>