staging: imx-drm: ipu-common: Remove unused variable

Fix the following warning when building with W=1 option:

drivers/staging/imx-drm/ipu-v3/ipu-common.c: In function 'ipu_remove':
drivers/staging/imx-drm/ipu-v3/ipu-common.c:1145:19: warning: variable 'res' set but not used [-Wunused-but-set-variable]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>