drm/amdgpu: make sure BOs are always kunmapped

When a BO is moved or destroyed it shouldn't be kmapped any more.

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