i965: Enable the ARB_transform_feedback_instanced extension on Gen7+.

This depends on ARB_transform_feedback2, so I've predicated it on the
ability to do register writes.

It also depends on ARB_transform_feedback3, which is the only reason we
couldn't expose it previously.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

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

@@ -231,6 +231,7 @@ intelInitExtensions(struct gl_context *ctx)
       if (can_do_pipelined_register_writes(brw)) {
          ctx->Extensions.ARB_transform_feedback2 = true;
          ctx->Extensions.ARB_transform_feedback3 = true;
+         ctx->Extensions.ARB_transform_feedback_instanced = true;
       }
    }