ALSA: hda - Integrate Digital Input Source to Input Source

STAC/IDT codecs provide both "Input Source" and "Digital Input Source"
controls to choose the analog input source and the digital input source.
But this is far user-unfriendly.

This patch merges the input source selections into one "Input Source"
control.  To have separate digital and analog input source controls,
you can pass "separate_dmux = 1 " hint string.

At the same time, this patch gets rid of analog mixer stuff that was
already disabled in previous patches.

Signed-off-by: Takashi Iwai <tiwai@suse.de>