drm/i915: vlv: setup RPS min/max frequencies once during init time

When enabling runtime PM on VLV, GT power save enabling becomes relatively
frequent, so optimize it a bit.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>