drm/i915: Remove unrequired POSTING_READ from gen6_set_rps()

The uncached mmio is sufficient to queue the mmio writes without raising
forcewake. The forced flush along with acquiring forcewake from the
posting read is not required for adjusting the RPS frequency.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170220094713.22874-3-chris@chris-wilson.co.uk


Reviewed-by: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>