drm/ttm: once more fix ttm_buffer_object_transfer

When the mutex is locked just in the moment we copy it we end up with a
warning that we release a locked mutex.

Fix this by properly reinitializing the mutex.

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