drm: drm_fops.c unlock missing on error path

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: Dan Carpenter <error27@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>