drm/stm: ltdc: remove clk_round_rate comment

Clk_round_rate returns rounded clock without changing
the hardware in any way.
This function couldn't replace set_rate/get_rate calls.
Todo comment has been removed & a new log inserted.

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1557500600-19771-1-git-send-email-yannick.fertre@st.com