drm/i915/skl: Updated the gen6_set_rps function

On SKL, the frequency is programmed differently in RPNSWREQ (A008)
register (from bits 23 to 31, compared to bits 24 to 31). So updated
the gen6_set_rps function, as per this change.

Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>