staging: drm/imx: check return value of ipu_reset()

ipu_reset() can fail with a timeout. Check the return value and act
appropriately.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>