staging: drm/imx: several bug fixes

- convert bogus IS_ERR_OR_NULL() to IS_ERR()
- fix copy/paste error
- check return value of ipu_crtc_init()

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>