To play a 3+ channels LPCM/DSD stream via HDMI,

	- HDMI sink must tell HDMI source about its speaker placements
	  (via ELD, speaker-allocation field)
	- HDMI source must tell the HDMI sink about channel allocation
	  (via audio infoframe, channel-allocation field)

(related docs: HDMI 1.3a spec section 7.4, CEA-861-D section 7.5.3 and 6.6)

This patch attempts to set the CA(channel-allocation) byte in the audio infoframe
according to
	- the number of channels in the current stream
	- the speakers attached to the HDMI sink

A channel_allocations[] line must meet the following two criteria to be
considered as a valid candidate for CA:
	1) its number of allocated channels = substream->runtime->channels
	2) its speakers are a subset of the available ones on the sink side

If there are multiple candidates, the first one is selected.  This simple
policy shall cheat the sink into playing music, but may direct data to the
wrong speakers.
Signed-off-by: Wu Fengguang <wfg@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>