ASoC: JZ4770: GCW0: Use "Headphone" instead of "AV-Out"

This matches the names in the codec. Now alsamixer will display
a combined widget for the headphones volume and mute.
ASoC: JZ4770: codec: Let the DAPM system take the DAC in and out of standby

For example when there is playback but all outputs are disabled, there
is no need to keep the DAC active.
ASoC: JZ4770: codec: Handle headphone power up/down in hpout_event()

Previously, the driver would update the hpout_enable flag and perform
the actual headphone power up/down sequence when playback was started
or stopped. As a result, often when plugging in the headphones when
playback had already started, there was no audible sound from the
headphones.