anv: fix AHB leak upon exportable allocation

anv: fix AHB leak upon exportable allocation

A successful AHardwareBuffer_allocate itself will increase a refcount on
the newly allocated AHB. For the import case, the implementation must
acquire a reference on the AHB. So if we layer the exportable allocation
on top of AHB allocation and AHB import, we must release an AHB
reference to avoid leak.

Signed-off-by: Yiwei Zhang <zzyiwei@chromium.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10940>