ALSA: hda - Use regmap for amp accesses

This patch converts the amp access functions to the regmap helpers.
The amp values were formerly cached in the own hash table.  Now it's
dropped by the regmap's cache.

The only tricky conversion is snd_hda_codec_amp_init().  This function
shouldn't do anything if the amp was already initialized.  For
achieving this behavior, a value is read once at first temporarily in
the cache-only mode.  Only if it returns an error,  i.e. the item
still doesn't exist in the cache, it proceeds to the update.

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