drm/i915: Print the hw context status is debugfs

Print out the HW context object information per ring. Even though the
existing code only utilizes the render ring, it's simple enough to
support future expansion.

I had this in a patch somewhere in a rev of the original implementation,
but I must have lost it.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: s/context/default context/ bikeshed applied.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>