drm/i915: Add more registers save/restore for Ironlake suspend

Add more display registers save/restore to fix unstable issues
during S4 testing on Ironlake. And DPLL_B_MD should not be restored
on Ironlake.

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