drm/mediatek: fixed the calc method of data rate per lane

Tune dsi frame rate by pixel clock, dsi add some extra signal (i.e.
Tlpx, Ths-prepare, Ths-zero, Ths-trail,Ths-exit) when enter and exit LP
mode, those signals will cause h-time larger than normal and reduce FPS.
So need to multiply a coefficient to offset the extra signal's effect.
  coefficient = ((htotal*bpp/lane_number)+Tlpx+Ths_prep+Ths_zero+
		 Ths_trail+Ths_exit)/(htotal*bpp/lane_number)

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>