drm/vkms: Add kerneldoc entry

Add an initial kerneldoc entry for vkms with a todo list.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
[danvet: Keep the todo.rst entry to point at the vkms docs instead.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180907174136.GA2648@haneenDRM