mesa: Implement ARB_shader_viewport_layer_array for i965

This extension is a combination of AMD_vertex_shader_viewport_index and
AMD_vertex_shader_layer, making it rather trivial to implement.

For gallium I *think* this needs a new cap because of the addition of
support in tessellation evaluation shaders, and since I don't have any
hardware to test it on, I've left that for someone else to wire up.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

docs/relnotes/12.1.0.html

@@ -53,6 +53,7 @@ Note: some of the new features are only available with certain drivers.
 <li>GL_ARB_indirect_parameters on radeonsi</li>
 <li>GL_ARB_shader_draw_parameters on radeonsi</li>
 <li>GL_ARB_shader_group_vote on nvc0</li>
+<li>GL_ARB_shader_viewport_layer_array on i965/gen6+</li>
 <li>GL_ARB_stencil_texturing on i965/hsw</li>
 <li>GL_ARB_texture_stencil8 on i965/hsw</li>
 <li>GL_EXT_window_rectangles on nv50, nvc0</li>