Currently only the DRM_MODE_REFLECT_Y rotation is supported. The driver
already supports reflection on the Y axis via a custom flag which is not
very useful because it requires custom userspace. Add the standard
rotation property that supports 0 degree rotation and Y axis reflection
for primary and overlay planes to provide a better interface than the
custom flag.

v2: keep custom flag for ABI compatibility (Dmitry)
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>