intel/perf: create a vtable for low-level driver functions

intel/perf: create a vtable for low-level driver functions

Performance metrics collections requires several actions (eg bo_map())
that have different implementations for Iris and i965.  The perf
subsystem needs a vtable for each of these actions, so it can invoke
the corresponding implementation for each driver.

The first call to be added to the table is bo_alloc.

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