drm/nouveau: Make use of TTM busy_placements.

Previously we were filling it the same as "placements", but in some
cases there're valid alternatives that we were ignoring completely.
Keeping a back-up memory type helps on several low-mem situations.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>