drm/i915: Read guaranteed freq for valleyview

Reading RP1 for valleyview to help us enable "pm_rps" i-g-t testcase
execution.

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
[danvet: Add missing static.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>