drm/amdgpu: fix amdgpu_bo_pin_restricted VRAM placing v2

commit 78d0e182b6c1f5336f6e8cbb197f403276dabc7f upstream.

We could pin BOs into invisible VRAM otherwise.

v2: make logic more readable as suggested by Michel

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com> (v1)
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>