drm/i915: Move intel_plane_atomic_calc_changes() & co. out

drm/i915: Move intel_plane_atomic_calc_changes() & co. out

Exfiltrate intel_plane_atomic_calc_changes() and its friends from
intel_display.c to intel_atomic_plane.c since that is a much better
fit.

While at it also nuke the official looking kernel docs for
intel_wm_need_update() and flag it for eventual destruction so
that people don't get any wrong ideas about using it in new code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220211090629.15555-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>