drm/amdgpu/ttm: move debugfs init into core amdgpu debugfs

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 ttm.

Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>