Let soc_dapm_mux_update_power() accept NULL as 'e' enum.

It makes the code a bit more robust and, more importantly, let the calling
mux force a disconnect of the output path if necessary.

This is useful if the dapm elements following the mux must be off
while updating the mux, to avoid glitches or force a (re)configuration.
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>