drm/kirin: Checking for IS_ERR() instead of NULL

The of_graph_get_remote_node() function doesn't return error pointers,
it returns NULL on error so I've updated the check.

Fixes: 86418f90 ("drm: convert drivers to use of_graph_get_remote_node")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171005125751.jvtjms62vbtxuvak@mwanda