drm/imx: don't touch primary fb on pageflip

The core already does the correct replacemet if the driver
page flip function returns without an error, so there is no
need to do it here.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>