staging: drm/imx: fix double free bug in error path

kfree(imx_drm_encoder) is already being called at the label
'err_register'.

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>