drm/i915: save/restore fence registers across suspend/resume

This makes software fallbacks not do tiling wrong on i965 and later after
resume. It also should fix 945 performance reduction after resume which
would have disabled tiling without causing any visible effect.

Signed-off-by: Keith Packard <keithp@keithp.com>
[anholt: Fixed up the 915 case to not save/restore the new regs]
Signed-off-by: Eric Anholt <eric@anholt.net>