drm/stm: ltdc: remove call of pm-runtime functions

It is not necessary to suspend or stop the ltdc clocks
to modify the pixel clock.

Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200701120402.6444-1-yannick.fertre@st.com