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: 8cc95f3f

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