drm/i915: Record the RING_MODE register for post-mortem debugging

Just another useful register to inspect following a GPU hang.

v2: Remove partial decoding of RING_MODE to userspace, be consistent and
use GEN > 2 guards around RING_MODE everywhere.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-32-git-send-email-chris@chris-wilson.co.uk