drm/i915/display: Remove dead code around intel_atomic_helper->free_list

After switching to directly using dma_fence instead of i915_sw_fence we
have left some dead code around intel_atomic_helper->free_list. Remove that
dead code.

v2: Remove intel_atomic_state->freed as well

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231114134141.2527694-1-jouni.hogander@intel.com