drm/i915: Add PM interrupt details and RPS thresholds to debugfs

When trying to determine whether RPS is working as intended, more
information is better. In particular, what interrupts are being
generated and the various thresholds for generating them.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>