drm: adp: Fix NULL vs IS_ERR() check in adp_plane_new()

The __drmm_universal_plane_alloc() function doesn't return NULL, it
returns error pointers.  Update the check to match.

Fixes: 332122eb ("drm: adp: Add Apple Display Pipe driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Sasha Finkelstein <fnkl.kernel@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/14a845e8-54d0-45f8-b8b9-927609d92ede@stanley.mountain


Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>