drm/ttm: enable swapout for reserved BOs during allocation

if the bo shares same reservation object then not lock it again
at swapout time to make it possible to swap out.

v2: refine the commmit message

Reviewed-by: Thomas Hellström <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chuming Zhou <david1.zhou@amd.com>
Signed-off-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>