drm/vc4: hvs: Store channel in variable

The assigned_channel field of our vc4_crtc_state structure is accessed
multiple times in vc4_hvs_atomic_flush, so let's move it to a variable
that can be used in all those places.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20220331143744.777652-4-maxime@cerno.tech