drm/panfrost: Fix a BO leak in panfrost_ioctl_mmap_bo()

We should release the reference we grabbed when an error occurs.

Fixes: 187d2929 ("drm/panfrost: Add support for GPU heap allocations")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191129135908.2439529-4-boris.brezillon@collabora.com