st/va: it is valid to release the VABuffer of an exported resource

st/va: it is valid to release the VABuffer of an exported resource

pipe_resource_reference(&res, NULL) will decrement reference counting,
i.e. p_atomic_dec(res->count). But the va surface still has the initial
reference since it has created the resource. So calling vaDestroyImage
on a derived image calls VaDestroyBuffer but the decrementation won't
reach 0. It is just wrong for vlVaDestroyBuffer to rely on the
export_refcount flag. Finally the vaapi intel driver has the same logic.

Signed-off-by: Julien Isorce <j.isorce@samsung.com>
Reviewed-by: Christian König <christian.koenig@amd.com>