drm/i915: Show support for accurate sw PMU busyness tracking

Expose whether or not we support the PMU software tracking in our
scheduler capabilities, so userspace can query at runtime.

v2: Use I915_SCHEDULER_CAP_ENGINE_BUSY_STATS for a less ambiguous
capability name.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703143702.11339-1-chris@chris-wilson.co.uk