As we allow for parallel threads to create the same vma instance
concurrently, and we only filter out the duplicates upon reacquiring the
spinlock for the rbtree, we have to free the loser of the constructors'
race. When freeing, we should also drop any resource references acquired
for the redundant vma.

Fixes: 2850748e ("drm/i915: Pull i915_vma_pin under the vm->mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: <stable@vger.kernel.org> # v5.5+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702083225.20044-1-chris@chris-wilson.co.uk