drm/dsi: Use peripheral's channel for DCS commands

When executing DCS commands, use the channel associated with the DSI
peripheral rather than one explicitly specified in the function call.
Devices shouldn't be able to step on each others' toes like this.

Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>