drm/panel: rm68200: allow using non-continuous dsi clock

The panel is able to work when dsi clock is non-continuous, thus
the system power consumption can be reduced using such feature.

Add MIPI_DSI_CLOCK_NON_CONTINUOUS to panel's mode_flags.

Signed-off-by: Antonio Borneo <antonio.borneo@st.com>
Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200918114736.11322-1-yannick.fertre@st.com