drm/fsl-dcu: use drm_display_mode_to_videomode to calculate timing parameters

Use the drm_display_mode_to_videomode function to calculate front/
back porches and sync length.

Cc: Madhur Verma <madhur.verma@samsung.com>
Cc: Hemanshu Srivastava <hemanshu.s@samsung.com>
Signed-off-by: Satendra Singh Thakur <satendra.t@samsung.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180926204150.29829-2-stefan@agner.ch