drm_open_helper() from drm_fops.c had a missing mutex_unlock in a error
path.

This was caught by smatch (http://repo.or.cz/w/smatch.git/).  Compile
tested.
Signed-off-by: NDan Carpenter <error27@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>