drm/i915/cdclk: have only one init/uninit function

While transitioning to having better clarity between the modules, it's
desirable to have the function name prefixes reflect the
module. Functions in intel_foo.c should be prefixed intel_foo_.

Expose only one CDCLK init/uninit function from intel_cdclk.c instead of
one per platform. Obviously this adds one "unnecessary" if ladder within
the entry points. However it should be considered more of a CDCLK
implementation detail how this is done per platform, instead of exposing
the fact. In other words, abstract the CDCLK module better.

No functional changes.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f63ed6e129098a32c63735be6cffa4756e7947af.1554461791.git.jani.nikula@intel.com