drm/panfrost: Put mapping instead of shmem obj on panfrost_mmu_map_fault_addr() error

When panfrost_mmu_map_fault_addr() fails, the BO's mapping should be
unreferenced and not the shmem object which backs the mapping.

Cc: stable@vger.kernel.org
Fixes: bdefca2d ("drm/panfrost: Add the panfrost_gem_mapping concept")
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220630200601.1884120-2-dmitry.osipenko@collabora.com