drm/sun4i: tcon: Make tcon_set_mux mode argument const

The drm_display_mode pointer can be mark const, so let's do it.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b0cce5a43fc3b56953d21a54fc3c14672f755f42.1508231063.git-series.maxime.ripard@free-electrons.com