drm/vc4: Simplify a bit the global atomic_check

When we can't allocate a new channel, we can simply return instead of
having to handle both cases, and that simplifies a bit the code.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201204151138.1739736-4-maxime@cerno.tech