drm/i915: implement new pm ops for i915

One problem in i915 hibernate with current legacy pci pm ops is
that after we do freeze, we'll be forced to do resume once again,
which re-init some resources and do modesetting again, that is
unnecessary for hibernate. This patch trys to bypass that.

We can't resolve this within legacy pm framework, but can do it
easily with new pm ops. Suspend (S3) process has also been kept
without change.

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