drm/cirrus-qemu: Remove custom plane state

Remove struct cirrus_primary_plane_state and its helpers, which
are all unused. Use struct drm_shadow_plane_state instead.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://lore.kernel.org/r/20250328091821.195061-5-tzimmermann@suse.de