drm/doc: link style-guide to doc-guide

Our style guide should only be the delta compared to the overall one.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1483008275-29383-1-git-send-email-daniel.vetter@ffwll.ch