drm/i915: HSW_BLC_PWM2_CTL doesn't exist on BDW

So don't write it, otherwise we will trigger unclaimed register
errors.

Testcase: igt/pm_rpm/rte
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>