drm/tegra: dc: Rename supports_blending to has_legacy_blending

Older Tegra chips do support blending as well. Rename the SoC info entry
.supports_blending to .has_legacy_blending to eliminate the confusion.

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