drm/i915: Use the cached min/min/rpe values in the vlv debugfs code

No need to re-read the hardware rps fuses when we already have all the
values tucked away in dev_priv->rps.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>