drm/i915: Add frame buffer compression support on Ironlake mobile

About 0.2W power can be saved on one HP laptop.

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>