drm/i915: Re-adjusting rc6 promotional timer for chv

After feedback from the hardware team we are changing the RC6
promotional timer to increase the power saving without
changing performance.

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>