drm/amdgpu: Don't warn on destroying a pinned BO

The warning turned out to be not so useful, as BO destruction tends to
be deferred to a workqueue.

Also, we should be preventing any damage from this now, so not really
important anymore to fix code doing this.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Mike Lothian <mike@fireburn.co.uk>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>