drm/i915: Fix legacy BLC event for pipe A

OpRegion event on 965G requires legacy BLC event enabled in pipe stat. As LVDS
could be on either pipe now, we should enable BLC event on both pipe. If fail to
do so, we couldn't handle the brightness request triggered from graphics
opregion.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>