drm/i915: Don't accidentally increase the frequency in handling DOWN rps

If we receive a DOWN_TIMEOUT rps interrupt, we respond by reducing the
GPU clocks significantly. Before we do, double check that the frequency
we pick is actually a decrease.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170210150348.22146-2-chris@chris-wilson.co.uk
Reviewed-by: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>