drm/i915: Show RPS autotuning thresholds along with waitboost

For convenience when debugging user issues show the autotuning
RPS parameters in debugfs/i915_rps_boost_info.

v2: Refine the presentation
v3: Style

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: fritsch@kodi.tv
Link: http://patchwork.freedesktop.org/patch/msgid/1471181336-27523-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471250973-31277-1-git-send-email-chris@chris-wilson.co.uk