drm/i915: report correct render clock frequencies on SNB

Fix up the debug file to report the right frequencies.  On SNB, we program
the PCU with a frequency ratio, which is multiplied by 100MHz on the CPU
side.  But GFX only runs at half that, so report it as such to avoid
confusion.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Keith Packard <keithp@keithp.com>