drm/i915: move dpll under display.dpll

Move display dpll related members 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>
Link: https://patchwork.freedesktop.org/patch/msgid/8818a2a4330edb9800f567626958b2de8872aa63.1661346845.git.jani.nikula@intel.com