drm: Handle dev->unique with drmm_

We need to add a drmm_kstrdup for this, but let's start somewhere.

This is not exactly perfect onion unwinding, but it's jsut a kfree so
doesn't really matter at all.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-21-daniel.vetter@ffwll.ch