drm/i915: Extract intel_post_plane_update_after_readout()

drm/i915: Extract intel_post_plane_update_after_readout()

Clean up the main commit_tail() codepath a bit by pulling
the post plane update steps that need to performed after
readout into their own little function
(intel_post_plane_update_after_readout()).

Declutters intel_atomic_commit_tail() a bit, and should
hopefully aid in keeping intel_pre_plane_update() vs.
intel_post_plane_update*() in sync.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240916152958.17332-3-ville.syrjala@linux.intel.com
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>