drm/i915: use 125MHz reference clock for PCH attached eDP

Fix the test so we don't try to use the 450MHz refclk on PCH attached
eDP.

References:
  https://bugs.freedesktop.org/show_bug.cgi?id=29141



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