drm/tegra: Introduce tegra_drm_submit()

Command stream submissions are the same across all devices that expose
a channel to userspace, so move the code into a generic function.

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

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

diff --cc drivers/gpu/drm/tegra/drm.h
Simple merge

diff --cc drivers/gpu/drm/tegra/gr2d.c
Simple merge