drm/i915: Use max(render, media) for Baytrail busyness calculation

Currently, we sum the render and media cycles (on different engines) to
compute a percentage - but we fail to factor in the duplication into the
threshold calculations. This makes us very eager to upclock!

If we just consider the maximum busy cycles of either counter, we should
have an accurate reflection on whether there are cycles to spare to
handle the workload at this frequency.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170309211232.28878-2-chris@chris-wilson.co.uk