drm/ttm: add common accounting to the resource mgr v3

It makes sense to have this in the common manager for debugging and
accounting of how much resources are used.

v2: cleanup kerneldoc a bit
v3: drop the atomic, update counter under lock instead

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com> (v1)
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Tested-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214093439.2989-2-christian.koenig@amd.com