intel/perf: move perf-related state into gen_perf_context

intel/perf: move perf-related state into gen_perf_context

To move more operations into intel/perf, several state items are
needed.  Save references to that state in the perf_ctxt, rather than
passing them in for every operation.

This commit includes an initializer for gen_perf_context, to set those
references and also encapsulate the initialization of the sample
buffer state.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>