anv/cmd_buffer: Always set up a null surface state

anv/cmd_buffer: Always set up a null surface state

We're about to start requiring it in yet another case and calculating
exactly when one is needed is starting to get prohibitively expensive.
A single surface state doesn't take up that much space so we may as well
create one all the time.

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>