gpu: host1x: Clean up debugfs on removal

The debugfs files created for host1x are never removed, causing these
files to be left dangling in debugfs. This results in a crash when any
of these files are accessed after the host1x driver has been removed,
as well as a failure to create the debugfs entries when they are added
again on driver probe.

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