drm/i915: make CS frequency read support missing more obvious

As suggested by Chris, we should make this more obvious for people
working with newer generations.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171213171154.6201-1-lionel.g.landwerlin@intel.com