drm/vc4: Convert existing documentation to actual kerneldoc.

I'm going to hook vc4 up to the sphinx build, so clean up its comments
to not generate warnings when we do.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227201144.10970-2-eric@anholt.net
(cherry picked from commit 72f793f1)