drm/imx/lcdc: fix a NULL vs IS_ERR() bug in probe

The devm_drm_dev_alloc() function returns error pointers.  It never
returns NULL.  Fix the check.

Fixes: c87e859c ("drm/imx/lcdc: Implement DRM driver for imx25")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d0a1fc55-3ef6-444e-b3ef-fdc937d8d57a@kili.mountain