amdgpu: fix race issue between two bo functions(v2)

there is race issue between two threads on amdgpu_bo_reference and
amdgpu_bo_import, this patch tends to fix it by moving the
pthread_mutex_lock out of bo_free_internal and move to bo_reference
to cover the update_reference part.

The mutex_unlock in bo_import should also cover bo refcount
increasement.

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