gpu: host1x: At first try a non-blocking allocation for the gather copy

The blocking gather copy allocation is a major performance downside of the
Host1x firewall, it may take hundreds milliseconds which is unacceptable
for the real-time graphics operations. Let's try a non-blocking allocation
first as a least invasive solution, it makes opentegra (Xorg driver)
performance indistinguishable with/without the firewall.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>