anv: Emit 3DSTATE_URB_* via a loop.

anv: Emit 3DSTATE_URB_* via a loop.

Rather than keeping separate {vs,hs,ds,gs}_start fields, we now store an
array indexed by the shader stage (MESA_SHADER_*).  The 3DSTATE_URB_*
commands are also sequentially numbered.  This makes it easy to just
emit them in a loop.

This simplifies the code a little, and also will make it easier to add
more credible HS and DS code later.