drm/amdgpu: Make amdgpu_vram_mgr_bo_invisible_size always accurate

Even BOs with AMDGPU_GEM_CREATE_NO_CPU_ACCESS may end up at least
partially in CPU visible VRAM, in particular when all VRAM is visible.

v2:
* Don't take VRAM mgr spinlock, not needed (Christian König)
* Make loop logic simpler and clearer.

Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>