drm/mediatek: Using the function drm_display_mode_to_videomode

This patch uses existing method drm_display_mode_to_videomode for
calculating front/back porches, sync lengths for mediatek dsi/dpi
drivers; instead of manually calculating them

Signed-off-by: Satendra Singh Thakur <thakursatendra2003@yahoo.co.in>
Signed-off-by: CK Hu <ck.hu@mediatek.com>