drm/i915: Extract register state error capture

The code has become quite hairy. By relocating all the generic registers
it will become more obvious where future ones should go. There is still
admittedly a bit of confusion left for things like per ring registers.

A subsequent patch will clean this function up.

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