drm/i915/dpll: hide struct intel_dpll_funcs

The struct is only needed in intel_dpll.c, move it there.

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/fedb27386cbad2b162d83a706273dd8ad9e8eed2.1643896905.git.jani.nikula@intel.com