drm/i915: Flush all writes before suspend

As we have already suspended the device, this should be a no-op except
for marking that all writes are indeed complete. The downside is that
we then have to walk all the lists of objects for what should be a no-op
(in some cases they will be mmio read to ensure the GGTT writes are
indeed flushed, and clflushes to ensure that cpu writes are in memory).

It seems prudent and the safer course for us to ensure all writes are
flushed to memory before suspend.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180601144125.18026-1-chris@chris-wilson.co.uk