drm/i915: Fix crash while aborting hibernation

Commit cbda12d7 (drm/i915: implement
new pm ops for i915) introduced the problem that if s2disk hibernation
is aborted, the system will crash, because i915_pm_freeze() does
nothing, while it should at least reverse some operations carried out
by i915_suspend().

Fix this issue by splitting the i915 suspend into a freeze part a
suspend part, where the latter is not executed before creating a
hibernation image, and the i915 resume into a "low-level" resume part
and a thaw part, where the former is not executed after the image has
been created.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Tested-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>