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>

drivers/gpu/drm/i915/i915_reg.h

@@ -1828,6 +1828,8 @@
  */
 #define   BACKLIGHT_DUTY_CYCLE_SHIFT		(0)
 #define   BACKLIGHT_DUTY_CYCLE_MASK		(0xffff)
+#define   BACKLIGHT_DUTY_CYCLE_MASK_PNV		(0xfffe)
+#define   BLM_POLARITY_PNV			(1 << 0) /* pnv only */
 
 #define BLC_HIST_CTL		0x61260