drm/cirrus: Inline cirrus_check_size() into primary-plane atomic_check

Inline the framebuffer size check into the primary plane's atomic_check
cirrus_primary_plane_atomic_check(). No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-15-tzimmermann@suse.de