drm/dsi: Implement some standard DCS commands

Add helpers for the {enter,exit}_sleep_mode, set_display_{on,off} and
set_tear_{on,off} DCS commands.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
[treding: kerneldoc and other minor cleanup]
Signed-off-by: Thierry Reding <treding@nvidia.com>