v3d: add missing unlock() in error path

CoverityID: 1435701
Fixes: e5a81ac7 ("broadcom/vc5: Don't forget to get the BO offset when opening a dmabuf.")
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <!5263>
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