nv50/ir: add OP_BUFQ for buffers query

nv50/ir: add OP_BUFQ for buffers query

TGSI RESQ allows both images and buffers but we have to make a
distinction between these two type of resources in our lowering pass.
Introducing OP_BUFQ which is a fake operand will allow to implement
OP_SUQ for surfaces.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>