drm/i915: fix i915_frequency_info on BDW

The GEN6_PM* registers don't exist on BDW anymore, so when we read
this file we trigger unclaimed register errors. The equivalent BDW
register for PMs is GEN8_GT_I*R(2), so use it.

Testcase: igt/pm_rpm/debugfs-read
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>