drm/i915: debugfs for context information

Currently this is only useful for the rc6 stuff. But this would also be
useful when I finally get around to the logical context + ppgtt stuff.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>