drm/tegra: dsi: Make FIFO depths host parameters

Rather than hardcoding them as macros, make the host and video FIFO
depths parameters so that they can be more easily adjusted if a new
generation of the Tegra SoC changes them.

While at it, set the depth of the video FIFO to the correct value of
1920 *words* rather than *bytes*.

Signed-off-by: Thierry Reding <treding@nvidia.com>