drm/tegra: sor: Fully initialize SOR before registration

Before registering the SOR host1x client, make sure that it is fully
initialized. This avoids a potential race condition between the SOR's
probe and the host1x device initialization in cases where the SOR is
the final sub-device to register to a host1x instance.

Reported-by: Jonathan Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>