drm/sun4i: tcon: Make tcon_get_clk_delay 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/e6f92f126640aa6de639386f9b4677db3d8bb37b.1508231063.git-series.maxime.ripard@free-electrons.com