drm/i915/skl: Retrieve the frequency limits

v2: Use the new function, gen6_init_rps_frequencies() (Damien)

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>