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>

src/intel/perf/gen_perf.h

@@ -188,6 +188,10 @@ struct gen_perf_config {
 
    /* Location of the device's sysfs entry. */
    char sysfs_dev_dir[256];
+
+   struct {
+      void *(*bo_alloc)(void *bufmgr, const char *name, uint64_t size);
+   } vtbl;
 };
 
 static inline size_t