ASoC: tegra20: i2s: Remove handing of disabled runtime PM

Runtime PM is always available on Tegra since commit 40b2bb1b


("ARM: tegra: enforce PM requirement"), hence there is no need to
handle the case of a disabled RPM by Tegra drivers. Remove handing
of a disabled runtime PM from Tegra20 I2S driver.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20210314154459.15375-12-digetx@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>