drm/tegra: firewall: Check for is_addr_reg existence in IMM check

In the IMM opcode check, don't call is_addr_reg if it's not set.

Fixes: 8cc95f3fd35e ("drm/tegra: Add job firewall")
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drm/tegra: Add job firewall

Add a firewall that validates jobs before submission to ensure
they don't do anything they aren't allowed to do, like accessing
memory they should not access.

The firewall is functionality-wise a copy of the firewall already
implemented in gpu/host1x. It is copied here as it makes more
sense for it to live on the DRM side, as it is only needed for
userspace job submissions, and generally the data it needs to
do its job is easier to access here.

In the future, the other implementation will be removed.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>