gpu: host1x: Resize channel register region on Tegra186 and later

The register region allocated per channel was decreased from 16384 bytes
to 256 bytes on Tegra186 and later. Resize the region to make sure every
channel (instead of only the first) is properly programmed.

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