drm/i915: do not access BLC_PWM_CTL2 on pre-gen4 hardware

The BLC_PWM_CTL2 register does not exist before gen4. While at it, do a
slight drive by cleanup of the code.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>