gpu: Only use one driver at a time

This allows to easily configure the wanted counters without having to rely on implementation details (like the internal number of the driver). The first argument will now be the used driver and the first available driver will be used in case it isn't specified.

To use several drivers, simply run the producer as many time as required.

This allows us to have per-driver configuration as changed in the gpu.cfg file.

Signed-off-by: Corentin Noël corentin.noel@collabora.com