drm/i915: add support for G41 chipset

This had been delayed for some time due to failure to work on the one piece
of G41 hardware we had, and lack of success reports from anybody else.
Current hardware appears to be OK.

Signed-off-by: Zhenyu Wang <zhenyu.z.wang@intel.com>
[anholt: hand-applied due to conflicts with IGD patches]
Signed-off-by: Eric Anholt <eric@anholt.net>