drm/atomic-helper: fix parameter order in drm_format_conv_state_copy() call

Old and new state parameters are swapped, so the old state was cleared
instead of the new duplicated state.

Fixes: 90367458 ("drm/atomic-helper: Add format-conversion state to shadow-plane state")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Leonard Göhrs <l.goehrs@pengutronix.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: <stable@vger.kernel.org> # v6.8+
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404081756.2714424-1-l.stach@pengutronix.de