drm/i915: Clamp efficient frequency to valid range

The efficient frequency (RPe) should stay in the range
RPn <= RPe <= RP0.  The pcode clamps the returned value
internally on Broadwell but not on Haswell.

Fix for missing range check in
commit 93ee2920
Author: Tom O'Rourke <Tom.O'Rourke@intel.com>
Date:   Wed Nov 19 14:21:52 2014 -0800

    drm/i915: Use efficient frequency for HSW/BDW

Reference: http://lists.freedesktop.org/archives/intel-gfx/2015-February/059802.html


Reported-by: Michael Auchter <a@phire.org>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org # v3.19
Signed-off-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>