anv: Treat zero size XFB buffer as disabled

anv: Treat zero size XFB buffer as disabled

Vulkan spec doesn't explicitly forbid zero size transform
feedback buffers.
Having zero size xfb caused SurfaceSize overflow and
triggered assert in debug build.

The only way to have zero size SO_BUFFER is to disable
SO_BUFFER as stated in hardware spec.

From SKL PRM, Vol 2a, "3DSTATE_SO_BUFFER":
  "If set, stream output to SO Buffer is enabled,
  if 3DSTATE_STREAMOUT::SO Function ENABLE is also enabled.
  If clear, the SO Buffer is considered "not bound" and effectively
  treated as a zero- length buffer for the purposes of SO output and
  overflow detection. If an enabled stream's Stream to Buffer Selects
  includes this buffer it is by definition an overflow condition.
  That stream will cause no writes to occur,
  and only SO_PRIM_STORAGE_NEEDED[<stream>] will increment."

Fixes: 36ee2fd61c8 "anv: Implement the basic form of VK_EXT_transform_feedback"
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>