drm/tegra: Use symbolic names for gr2d registers

Instead of using magic numbers for the registers which contain memory
addresses in the firewall table, using symbolic names.

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

diff --cc drivers/gpu/drm/tegra/gr2d.c
Simple merge

diff --cc drivers/gpu/drm/tegra/gr2d.h
Simple merge