drm/amdgpu: Keep track of amount of pinned CPU visible VRAM

Instead of CPU invisible VRAM. Preparation for the following, no
functional change intended.

v2:
* Also change amdgpu_vram_mgr_bo_invisible_size to
  amdgpu_vram_mgr_bo_visible_size, allowing further simplification
  (Christian König)

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