drm/nouveau: stop using TTM placement flags

Those are going to be removed, stop using them here.

Instead use the GEM flags from the UAPI.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/389825/?series=81551&rev=1