drm/amdgpu/display: split dp connector registration (v4)

Split into init and register functions to avoid a segfault
in some configs when the load/unload callbacks are removed.

v2:
- add back accidently dropped has_aux setting
- set dev in late_register

v3:
- fix dp cec ordering

v4:
- squash in kdev reference fix

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>