drm/i915: use i915_gem_object_put_unlocked() after releasing mutex

The exit path in intel_overlay_put_image_ioctl() first unlocks the
struct_mutex, then drops its reference to 'new_bo' by calling
i915_gem_object_put(). As it isn't holding the mutex at this point,
this should be i915_gem_object_put_unlocked().

This was previously correct but got splatted in the recent
s/drm_gem_object_unreference/i915_gem_object_put/
where the _unlocked suffix was lost in this one case.

v2: don't bother fixing whitespace glitch [Chris Wilson]
    Chris can do it next time he touches gem_evict.c ;)

Fixes: f8c417cd

 drm/i915: Rename drm_gem_object_unreference in preparation ...
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469122778-14416-1-git-send-email-david.s.gordon@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>