drm/tegra: dc: Registers are 32 bits wide

drm/tegra: dc: Registers are 32 bits wide

Using an unsigned long type will cause these variables to become 64-bit
on 64-bit SoCs. In practice this should always work, but there's no need
for carrying around the additional 32 bits.

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