gpu: Improve driver interface

The `dump_perfcnt()` function does not return a timestamp associated to a
record anymore. This is due to the fact that different GPUs might report
more than one record. We therefore introduce a `next()` function which
iterates through the samples.

Signed-off-by: Antonio Caggiano <antonio.caggiano@collabora.com>
Reviewed-by: Rohan Garg <rohan.garg@collabora.com>
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