drm/i915: move dpll_funcs to display.funcs

Move display dpll functions under drm_i915_private display sub-struct.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5bf7aada2c7f0500c5002fa089fb66a92019a301.1661346845.git.jani.nikula@intel.com