drm/tegra: dc: Support background color

Starting with Tegra124, the interface to set the background color (the
value generated for pixels that are not sourced from any window) is via
a different register. Earlier generations called this the border color.
Reverse the feature flag and assume that IP revisions that don't have
support for background color will support border color instead.

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