i965: Guard access to gl_Layer by extension #ifdef

Only assign gl_Layer if we have GL_AMD_vertex_shader_layer.  Gen6 doesn't
(currently) have that extension, but it also doesn't support layered
rendering.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Jordan Justen <jordan.l.justen@intel.com>

src/mesa/drivers/dri/i965/brw_meta_fast_clear.c

@@ -97,7 +97,9 @@ brw_bind_rep_write_shader(struct brw_context *brw, float *color)
       "uniform int layer;\n"
       "void main()\n"
       "{\n"
+      "#ifdef GL_AMD_vertex_shader_layer\n"
       "   gl_Layer = gl_InstanceID;\n"
+      "#endif\n"
       "   gl_Position = position;\n"
       "}\n";
    const char *fs_source =