amd_iommu: fix lock imbalance

In alloc_coherent there is an omitted unlock on the path where mapping
fails. Add the unlock.

[ Impact: fix lock imbalance in alloc_coherent ]

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>