drm/i915: add the FCLK case to intel_ddi_get_cdclk_freq

We already have code to disable LCPLL and switch to FCLK, so we need this too.
We still don't call the code to disable LCPLL, but we'll call it when we add
support for Package C8+.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>