drm/atomic-helper: Fix commit_tail state variable name

Even though the commit_tail () drm_atomic_state parameter is called
old_state, it's actually the state being committed which is confusing.

It's even more confusing since the atomic_commit_tail hook being called
by commit_tail() parameter is called state.

Let's rename the variable from old_state to state to make it less
confusing.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20250213-bridge-connector-v3-6-e71598f49c8f@kernel.org


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