gpu: host1x: Show number of pending waiters in debugfs

Show the number of pending waiters in the debugfs status file.
This is useful for testing to verify that waiters do not leak
or accumulate incorrectly.

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