drm/i915: Tidy i915_gem_suspend()

In the next patch, we will make a fairly minor change to flush
outstanding resets before suspend. In order to keep churn to a minimum
in that functional patch, we fix up the comments and coding style now.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709130208.11730-7-chris@chris-wilson.co.uk