drm/doc: Update errno handbook

We recently bikeshedded this to a different flavour, but forgot to
update the recommendations.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181019084311.23836-1-daniel.vetter@ffwll.ch