headers: Sync up some header guard changes from drm-next.

This pulls in pieces of drm-next d65d31388a23
("Merge tag 'drm-misc-next-fixes-2017-11-07' of
git://anongit.freedesktop.org/drm/drm-misc

 into drm-next")

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>