drm/i915: initialize gen6 rps work queue on Sandy Bridge and Ivy Bridge

It's not used on Ironlake, but is used on later generations, so make
sure it exists before we try to use it in the interrupt handlers.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>