drm/stm: dsi: higher pll out only in video burst mode

In order to better support video non-burst modes,
the +20% on pll out is added only in burst mode.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1568278589-20400-1-git-send-email-yannick.fertre@st.com