drm/i915: Fix calculation of backlight value in combined mode

commit ba3820ade317ee36e496b9b40d2ec3987dd4aef0 upstream.

[The upstream commit above is a combination of revert + one-liner fix.
 This patch contains only the latter that is needed for 2.6.37.x.]

This patch fixes the backlight level calculation for combination mode
used in some models like GM45.  It's due to a wrong bit shift
introduced by the commit a95735569312f2ab0c80425e2cd1e5cb0b4e1870
    drm/i915: Refactor panel backlight controls

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=23472
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=34524
Bugzilla: https://bugzilla.novell.com/show_bug.cgi?id=672946

Signed-off-by: Takashi Iwai <tiwai@suse.de>