drm/dsi: Add DSI transfer helper

A common pattern is starting to emerge for higher level transfer
helpers. Create a new helper that encapsulates this pattern and avoids
code duplication.

Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>