drm/i915: Remove posting-read for forcewake put

We can relax the requirement upon ourselves that the forcewake is
released immediately and just allow it to occur naturally following our
mmio request.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323101944.21627-5-chris@chris-wilson.co.uk