drm/i915/dpll: make intel_shared_dpll_funcs internal to intel_dpll_mgr.c

Move struct intel_shared_dpll_funcs to intel_dpll_mgr.c, as no other
place needs to have access to it. We also don't need to have kernel-doc
documentation for file internal structures, so drop them while at it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220119110528.2377899-1-jani.nikula@intel.com