drm/dsi: Implement DCS nop command

Provide a small convenience wrapper that transmits a DCS nop command.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>

diff --cc drivers/gpu/drm/drm_mipi_dsi.c
Simple merge

diff --cc include/drm/drm_mipi_dsi.h
Simple merge