gpu: host1x: debug: Fix multiple channels emitting messages simultaneously

Once channel's job is hung, it dumps the channel's state into KMSG before
tearing down the offending job. If multiple channels hang at once, then
they dump messages simultaneously, making the debug info unreadable, and
thus, useless. This patch adds mutex which allows only one channel to emit
debug messages at a time.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>