drm/i915: make intel_dpll_mgr.h self-contained

And ensure it stays that way.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8e759b36dee10c20aa06e4d34d36cda6a2a02323.1556540889.git.jani.nikula@intel.com