drm/panel: rm68200: fix mode to 50fps

Compute new timings to get a framerate of 50fps with a pixel clock
@54Mhz.

Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200925141618.12097-1-yannick.fertre@st.com