drm/amdgpu: Refactor amdgpu_vram_mgr_bo_invisible_size helper

commit 5e9244ff585239630f15f8ad8e676bc91a94ca9e upstream.

Preparation for the following fix, no functional change intended.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>