drm/i915: fix up PCH backlight #define mixup

I so totally suck.

This can cause a black screen if (for whatever reason) the bios
hasn't set this bit itself.

This regression has been introduced in

commit 7cf4160148136deb31ee5f2802857dd935a38529
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Jun 5 10:07:09 2012 +0200

    drm/i915: clear up backlight #define confusion on gen4+

Tested-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -1900,7 +1900,7 @@
 /* PCH CTL1 is totally different, all but the below bits are reserved. CTL2 is
  * like the normal CTL from gen4 and earlier. Hooray for confusing naming. */
 #define BLC_PWM_PCH_CTL1	0xc8250
-#define   BLM_PCH_PWM_ENABLE			(1 << 30)
+#define   BLM_PCH_PWM_ENABLE			(1 << 31)
 #define   BLM_PCH_OVERRIDE_ENABLE		(1 << 30)
 #define   BLM_PCH_POLARITY			(1 << 29)
 #define BLC_PWM_PCH_CTL2	0xc8254