videonode: imx6: clear texture cache when the format changes

The old textures won't match anyways. So there is no need to keep them.

Change-Id: Id3482333d10cf022d04076ec0f5c7df475c522ae
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>