drm/i915: Avoid setting ring freq on invalid rps freqs

Looping through rps frequencies when both min and max are zero
ends up into an endless loop. This can happen during hardware
enablement.

Bail out early if rps frequencies are not correctly set yet.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180320151734.11761-1-mika.kuoppala@linux.intel.com