gpu: host1x: Check reloc table before usage

The firewall assumed that the user space always delivers a relocation
table when it is accessing address registers. If userspace did not
deliver a relocation table and tried to access the address registers,
the code performed bad memory accesses.

This patch modifies the firewall to check correctly that the firewall
table is available before accessing it. In addition, check_reloc() is
converted to use boolean return value (true when the reloc is valid,
false when invalid).

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