gpu: host1x: Register context bus unconditionally

Conditional registration is a problem for other subsystems which may
unwittingly try to interact with host1x_context_device_bus_type in an
uninitialised state on non-Tegra platforms. A look under /sys/bus on a
typical system already reveals plenty of entries from enabled but
otherwise irrelevant configs, so lets keep things simple and register
our context bus unconditionally too.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>