drm/i915: Logically reorder error register capture

Create logical sections in an attempt to clean up, and continue to keep
future additions clean.

v2: Reworded the comments. Added section headers (Chris)

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>