drm/ttm: drop persistent_swap_storage from ttm_bo_init and co

Never used as parameter, the only driver actually using this is nouveau
and there it is initialized after the BO is initialized.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>