drm/vgem: Drop dev->struct_mutex

With the previous two changes it doesn't protect anything any more.

v2: Use _unlocked unreference variant.

v3: Appease gcc noise.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-13-git-send-email-daniel.vetter@ffwll.ch