drm/tegra: dsi: Add command mode support

Add support for DC-driven command mode. This is a mode where the video
stream sent by the display controller is packed into DCS command packets
(write_memory_start and write_memory_continue) by the DSI controller. It
can be used for panels with a remote framebuffer and is useful to save
power when used with a dynamic refresh rate (not yet supported by the
driver).

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