drm/ttm: export ttm_pool_fini for cleanup

ttm_pool_init is exported and used outside of ttm subsystem with
amdgpu_ttm interface, similarly export ttm_pool_fini for proper cleanup.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>