drm/simpledrm: Preallocate format-conversion buffer in atomic_check

Preallocate the format-conversion state's storage in the plane's
atomic_check function if a format conversion is necessary. Allows
the update to fail if no memory is available. Avoids the same
allocation within atomic_update, which may not fail.

Also inline drm_plane_helper_atomic_check() into the driver and thus
return early for invisible planes. Avoids memory allocation entirely
in this case.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231009141018.11291-6-tzimmermann@suse.de