drm/imx/dcss: Don't call dev_set_drvdata(..., NULL);

The driver core takes care about removing driver data, so this can be
dropped from the driver.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Laurentiu Palcu <laurentiu.palcu@oss.nxp.com>
Signed-off-by: Laurentiu Palcu <laurentiu.palcu@oss.nxp.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221230130025.240776-2-u.kleine-koenig@pengutronix.de