drm/ttm: export memory type debug entrypoint.

As suggested on review, just export the memory type debug for
drivers to use, while also making the debug callback optional
(don't need to test for system as it won't init it).

rename it to be more consistent with object name for now.
(we may rename all the objects later.)

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-9-airlied@gmail.com