Driver timestamp and sampling period

Improve driver interface by:

• Passing sampling period as an argument when enabling performance counters. Even though PanfrostDriver does not use it, IntelDriver needs this value for a proper initialization.

• Returning a timestamp when dumping performance counters. PanfrostDriver just calculates a timestamp and returns it, but IntelDriver process perf records asynchronously, and these records have their own GPU timestamp which should be used instead.