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

Looks like a better place for this.

v2: use atomic64_t members 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>