drm/bridge: tc358768: Use struct videomode

The TC358768 documentation uses HFP, HBP, etc. values to deal with the
video mode, while the driver currently uses the DRM display mode
(htotal, hsync_start, etc).

Change the driver to convert the DRM display mode to struct videomode,
which then allows us to use the same units the documentation uses. This
makes it much easier to work on the code when using the TC358768
documentation as a reference.

Reviewed-by: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Tested-by: Maxim Schwalm <maxim.schwalm@gmail.com> # Asus TF700T
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230906-tc358768-v4-6-31725f008a50@ideasonboard.com