This fixes piglit/glsl-vs-main-return and glsl-fs-main-return for the drivers
which don't support RET (i915g, r300g, r600g, svga).

ir_to_mesa does not currently generate subroutines, but it's a matter of time
till it's added. It would then break all the drivers which don't implement
them, so this CAP makes sense.

Signed-off-by: Marek Olšák <maraeo@gmail.com>