Add busy and busy-avg balancers which make balancing decisions by looking
at engine busyness via the i915 PMU.

And thus are able to make decisions on the actual instantaneous load of
the system, and not use metrics that lag behind by a batch or two. In
doing so, each client should be able to greedily maximise their own
usage of the system, leading to improved load balancing even in the face
of other uncooperative clients. On the other hand, we are only using the
instantaneous load without coupling in the predictive factor for dispatch
and execution length.

v2:
 * Commit text. (Chris Wilson)
 * Rename get_stats to get_pmu_stats. (Chris Wilson)
 * Fix PMU readout in VCS remap mode.

v3:
 * Integrated Petri's meson build recipe.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Petri Latvala <petri.latvala@intel.com>