gpu: host1x: Check for valid host1x pointer

Under rare circumstances it can happen that the host1x driver's .probe()
doesn't finish properly, in which case the device's driver-specific data
will not be set. Instead of crashing in such a situation, propagate the
error to callers of the host1x_get_drm_data() function.

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