gpu: host1x: Don't reset firewall between gathers

The firewall was reinitialised for each gather. Because the filter
was reinitialised, it did not track the class over gather boundaries.
This allowed the user application to set host1x class to one class
in one gather and use that class in another gather without firewall
having knowledge about that.

Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>