iris: Fall back to fresh allocations of mapping for zero-memset fails.

iris: Fall back to fresh allocations of mapping for zero-memset fails.

It is unlikely that we would fail to map a cached BO in order to zero
its contents.  When we did, we would free the first BO in the cache and
try again with the second.  It's possible that this next BO already had
a map setup, in which case we'd succeed.  But if it didn't, we'd likely
fail again in the same manner.

There's not much point in optimizing this case (and frankly, if we're
out of CPU-side VMA we should probably dump the cache entirely)...so
instead, just fall back to allocating a fresh BO from the kernel which
will already be zeroed so we don't have to try and map it.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>