drm/i915: collect more per ring error state

Based on a patch by Ben Widawsky, but with different colors
for the bikeshed.

In contrast to Ben's patch this one doesn't add the fault regs.
Afaics they're for the optional page fault support which
- we're not enabling
- and which seems to be unsupported by the hw team. Recent bspec
  lacks tons of information about this that the public docs released
  half a year back still contain.

Also dump ring HEAD/TAIL registers - I've recently seen a few
error_state where just guessing these is not good enough.

v2: Also dump INSTPM for every ring.

v3: Fix a few really silly goof-ups spotted by Chris Wilson.

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>