drm/i915: Fix resume regression on MSI Wind U100 w/o KMS

Commit cbda12d7 (drm/i915: implement
new pm ops for i915), among other things, removed the .suspend and
.resume pointers from the struct drm_driver object in i915_drv.c,
which broke resume without KMS on my MSI Wind U100.

Fix this by reverting that part of commit cbda12d7.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[anholt: added comment explaining when .suspend/.resume matter]
Signed-off-by: Eric Anholt <eric@anholt.net>