drm/vc4: Add a paragraph at the top of vc4 docs introducing what it is.

This makes for more sensible documentation of the whole module than
jumping straight into the details of display.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227201144.10970-5-eric@anholt.net