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

drm_atomic_helper_commit_tail_rpm() is the final part of an atomic
commit, and is given the state being committed as a parameter.

However, that parameter is named old_state, but documented as the "new
modeset state" which is all super confusing.

Let's rename that parameter to state.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20250213-bridge-connector-v3-9-e71598f49c8f@kernel.org


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