drm/i915: Skip force-wake for uncached mmio flush of GGTT writes

The trick of using an uncached mmio read to ensure that the GGTT writes
are flushed does not require us to do the forcewake dance, so avoid it
in the hope of reducing the frequency that we do keep the device forced
awake.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170318104257.694-1-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>