drm/i915/cdclk: make struct intel_cdclk_state opaque

With all the code touching struct intel_cdclk_state moved inside
intel_cdclk.c, we move the struct definition there too, and make the
type opaque. This nicely reduces includes from intel_cdclk.h.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/2b58c52e8cbcb66a48ecd4a1453e49dc7bd66289.1750847509.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>