drm: vm: Clean up documentation

Fix kernel doc comments to avoid warnings when compiling with W=1.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200306102937.4932-1-benjamin.gaignard@st.com