drm/i915: Extract intel_atomic_swap_state()

drm/i915: Extract intel_atomic_swap_state()

Pull all the state swap stuff into its own function to declutter
intel_atomic_commit() a bit.

Note that currently the state swap is spread across both
sides of the unprepare branch in intel_atomic_commit(), but
we can pull all of it ahead a bit since we bail on the first
error, and thus there is no change in behaviour from the
reordering.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231219130756.25986-4-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>