drm/amdgpu: move gtt usage tracking into the gtt manager v2

It doesn't make much sense to count those numbers twice.

v2: use and atomic64_t instead

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>