ASoC: dapm: Ignore VMID widgets for target bias

VMID widgets behave very similar to signal generator widgets. Both are always
considered to be powered up. This means that we need to ignore the VMID widgets
in the same way as signal generator widgets when calculating the DAPM context's
target bias level. Otherwise the presence of a VMID widget, regardless whether
it is on an active path or not, will cause the DAPM context to be powered up.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>