drm/tegra: dc: Restore YUV overlay support

Commit ebae8d07

 ("drm/tegra: dc: Implement legacy blending") broke
support for YUV overlays by accident. The reason is that YUV formats are
considered opaque because they have no alpha component, but on the other
hand no corresponding format with an alpha component can be returned. In
the case of YUV formats, the opaque format is the same as the alpha
format, so add the special case to restore YUV overlay support.

Reported-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>