gpu: pvr: improve per process procfs entry/dir handling

gpu: pvr: improve per process procfs entry/dir handling

When building with full debug, a lot of "Resource Arena" (ra) data is
being made available in the rather painful procfs, including per process
ra info, kept in process specific subdirectories.

Adding and removing process specific entries from procfs took the PID of
the currently running process; which sometimes failed during cleanup,
when the current process might no longer be anything tracked by the
driver. This then resulted in some strange behaviour, where it was
impossible to cleanup a process specific procfs directory, resulting
in an endless loop.

Since process specific procfs entries are only created in the RA code,
we now track the pid in the RA struct. When we pass this pid to
both the procfs entry creation and removal functions, we now do
reliably clean up the procfs entries.

Signed-off-by: Luc Verhaegen <libv@codethink.co.uk>
Signed-off-by: Imre Deak <imre.deak@nokia.com>