drm/amdgpu/display: move debugfs init into core amdgpu debugfs (v2)

In order to remove the load and unload drm callbacks,
we need to reorder the init sequence to move all the drm
debugfs file handling.  Do this for display.

v2: add config guard for DC

Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Harry Wentland <harry.wentland@amd.com> (v1)
Acked-by: Christian König <christian.koenig@amd.com> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>