drm/i915: pnv has a backlight polarity control bit, too

We already correctly ignore bit0 on gen < 4, now we also know why ;-)
I've decided that losing that single bit of precision isn't worth the
trouble to sprinkle IS_PINEVIEW checks all over the backlight control
code - that code is way too fragile imo.

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>