drm/i915: fix pnv display core clock readout out

We need the correct clock to accurately assess whether we need to
enable the double wide pipe mode or not.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Stéphane Marchesin <marcheu@chromium.org>
Cc: Stuart Abercrombie <sabercrombie@google.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>