drm/i915: refactor ring error state capture to use arrays

The code already got unwieldy and we want to dump more per-ring
registers.

Only functional change is that we now also capture the video
ring registers on ilk.

v2: fixup a refactor fumble 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>