st/mesa: add a fallback for clear_with_quad when no vs_layer

st/mesa: add a fallback for clear_with_quad when no vs_layer

Not all drivers can set gl_Layer from VS. Add a fallback that passes the
instance id from VS to GS, and then uses the GS to set the layer.

Tested by adding

  quad_buffers |= clear_buffers;
  clear_buffers = 0;

to the st_Clear logic, and forcing set_vertex_shader_layered in all
cases. No piglit regressions (on piglits with 'clear' in the name).

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: "10.4 10.3" <mesa-stable@lists.freedesktop.org>