drm/doc: Update styleguide

The new cool is &struct foo (kernel-doc now copes with linebreaks),
and structure members should be referenced using &foo.bar.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483044517-5770-8-git-send-email-daniel.vetter@ffwll.ch