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

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

drm_atomic_helper_commit_writebacks() updates all writeback connectors
affected by a new commit. It takes the drm_atomic_state being committed
as a parameter.

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-v3-20-e71598f49c8f@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>