gpu: host1x: debug: Dump push buffer state

When job hangs and there is a memory error pointing at channel's push
buffer, it is very handy to know the push buffer's state. This patch
makes the push buffer's state to be dumped into KMSG in addition to the
job's gathers.

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