drm/i915: Add local DPLL 'hw_state' variables

Add some local 'hw_state' variables to the old DPLL code.
Will help with unionizing the dpll_hw_state later.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240412182703.19916-16-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>