drm/atomic-helper: Change parameter name of drm_atomic_helper_commit_tail()

drm_atomic_helper_commit_tail() is the final part of an atomic commit,
and is given a parameter with the drm_atomic_state being committed.

However, that parameter name is called (and documented) as old_state,
which is pretty confusing. Let's rename that variable as state.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20250213-bridge-connector-v3-8-e71598f49c8f@kernel.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>