ASoC: tegra: utils: Don't use of_have_populated_dt()

Recent list discussions concluded that drivers should not be calling
of_have_populated_dt(), and hence of_have_populated_dt() should not be
exported. Use a different mechanism to detect DT vs. non-DT boot.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>