drm: Distinguish no name from ENOMEM in set_unique()

If passing name == NULL to drm_drv_set_unique() we now get -ENOMEM
as kstrdup() returns NULL. Instead check for this explicitly and
return -EINVAL if no name is provided.

Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160921145919.13754-1-teg@jklm.no