drm/amdgpu: warn when there are still mappings when a BO is destroyed v2

This can only happen when there is a reference counting bug.

v2: fix typo

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>