drm/i915: Also drop vm.ref along error paths for vma construction

drm/i915: Also drop vm.ref along error paths for vma construction

Not only do we need to release the vm.ref we acquired for the vma on the
duplicate insert branch, but also for the normal error paths, so roll
them all into one.

Reported-by: Andi Shyti <andi.shyti@intel.com>
Suggested-by: Andi Shyti <andi.shyti@intel.com>
Fixes: 587eefbe9921 ("drm/i915: Pull i915_vma_pin under the vm->mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: <stable@vger.kernel.org> # v5.5+
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702211015.29604-1-chris@chris-wilson.co.uk
(cherry picked from commit 03fca66b7a36b52da8915341eee388267f6d5b73)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>