drm/amdgpu: Don't move pinned BOs

The purpose of pinning is to prevent a buffer from moving.

Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>