drm: zte: checking for NULL instead of IS_ERR()

drm_dev_alloc() never returns NULL, it only returns error pointers on
error.

Fixes: 0a886f59 ("drm: zte: add initial vou drm driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161115095301.GC15424@mwanda