drm/vc4: Add RST to bring together vc4 kerneldoc.

This doesn't yet produce coherent documentation of the module, but at
least gets the kerneldoc built and somewhat glued together.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227201144.10970-3-eric@anholt.net