nouveau: use the buffer usage to determine placement when no binding

nouveau: use the buffer usage to determine placement when no binding

With ARB_direct_state_access, buffers can be created without any binding
hints at all. We still need to allocate these buffers to VRAM or GART,
as we don't have logic down the line to place them into GPU-mappable
space. Ideally we'd be able to shift these things around based on usage.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92438
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>