drm: fix parameters documentation style in drm_dma

Remove old documentation style and use new one to avoid warnings when
compiling with W=1

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200114160135.14990-1-benjamin.gaignard@st.com