drm/ttm: Make sure BOs being swapped out are cacheable

The current caching state may not be tt_cached, even though the
placement contains TTM_PL_FLAG_CACHED, because placement can contain
multiple caching flags. Trying to swap out such a BO would trip up the

	BUG_ON(ttm->caching_state != tt_cached);

in ttm_tt_swapout.

Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com&gt;.>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Christian König <christian.koenig@amd.com>