drm: atomic helper: fix W=1 warnings

Few for_each macro set variables that are never used later which led
to generate unused-but-set-variable warnings.
Add (void)(foo) inside the macros to remove these warnings

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191008124254.2144-1-benjamin.gaignard@st.com