drm/i915: Ironlake suspend/resume support

This adds registers save/restore for Ironlake to make suspend work.

Signed-off-by: Guo, Chaohong <chaohong.guo@intel.com>
[zhenyuw: some code re-orgnization, and add more save/restore for
FDI link and transcoder registers, also fix palette register for Ironlake]
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>