drm/tegra: plane: Fix RGB565 format on older Tegra

The opaque/alpha format conversion code is currently only looking at
XRGB formats because they have an equivalent ARGB format. The opaque
format for RGB565 is RGB565 itself, much like the YUV formats map to
themselves.

Reported-by: Dmitry Osipenko <digetx@gmail.com>
Fixes: ebae8d07 ("drm/tegra: dc: Implement legacy blending")
Signed-off-by: Thierry Reding <treding@nvidia.com>