drm/stm: ltdc: restore calls to clk_{enable/disable}

Restore calls to clk_{enable/disable} deleted after applying the wrong
version of the patch

Fixes: fd6905fc ("drm/stm: ltdc: remove clk_round_rate comment")

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190527115830.15836-1-benjamin.gaignard@st.com