drm/modes: tag unused variables to avoid warnings

Some variables are set but never used. To avoid warning when compiling
with W=1 and keep the algorithm like it is tag theses variables
with _maybe_unused macro.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210102437.19377-1-benjamin.gaignard@st.com