drm/i915: Limit CHV max cdclk

Limit CHV maximum cdclk to 320MHz.

v2: Rebase to the latest
v3: Clean up of if-else tree

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>