drm/vc4: kms: Store channel in local variable

We use the channel from our vc4_crtc_state structure in multiple places,
let's store it in a local variable to make it cleaner.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20220328153659.2382206-5-maxime@cerno.tech