drm/i915: Split the CDCLK retrieval per-platform

This is only going to get worse, so split it now to avoid adding more
cases to the if/else ladder.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>