drm/nouveau: remove allocations from gart populate() hook

Since some somewhat questionable changes a while back, TTM provides a
completely empty array of struct dma_address that stays around for the
entire lifetime of the TTM object.

Lets use this array, *always*, rather than wasting yet more memory on
another array who's purpose is identical, as well as yet another bool array
of the same size saying *which* of the previous two arrays to use...

This change will also solve the high order allocation failures seen by
some people while using nouveau.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>