drm/i915: Don't spam dmesg with rps messages on vlv/chv

If the GPU frequency isn't going to change don't spam dmesg with
debug messages about it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>