drm/atmel-hlcdc: Rename custom plane state variable

Subsequent reworks will pass the global atomic state in the function
prototype, and atomic_check and atomic_update already have such a
variable already. Let's change them to ease the rework.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210219120032.260676-3-maxime@cerno.tech