nouveau: use ttm populate mapping functions. (v2)

Instead of rolling driver copies of them.

v2: cleanup return handling (Ben)

Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-4-airlied@gmail.com