drm/i915/display: use to_intel_display() global state macros

Convert intel_atomic_get_{old,new}_cdclk_state() and
intel_atomic_get_{old,new}_dbuf_state() to use to_intel_display()
instead of to_i915().

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240904125301.3813721-1-jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>