drm/i915: Emit telltales for extra levels of debug upon initialisation

After printing our welcome message to the user, also include
supplementary details on what debugging is enabled (useful for us to
sanity check what extra safeguards are on for any random kernel).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161014132707.29039-1-chris@chris-wilson.co.uk


Reviewed-by: Matthew Auld <matthew.auld@intel.com>