ASoC: soc-compress: Provide a runtime for the compressed FE substream

When a compressed stream is used on the FE side of a DPCM link, the
BE is still running as 'normal' PCM.

DPCM lends the runtime of the FE to every BE it opens and
dpcm_be_reparent() re-points it at another FE when the lending one
disconnects. The internal PCM created for a compressed FE by
snd_soc_new_compress() is never opened via the PCM API, so its
substream has no runtime and the BEs of a compressed FE were left
with a NULL one.

BE DAI and codec drivers can look at substream->runtime, they add
their constraints to it in startup(), for example cs42l43, cs42l42,
hdac_hdmi.

The other users of compressed DPCM do not hit this as their BEs are
DSP internal ports, driven by code which takes everything from the
hw_params it is passed and never looks at substream->runtime. The
BEs here end at generic CODEC drivers which are shared with the non
DPCM case and cannot be expected to know about the FE type.

Allocate a runtime for the compressed FE substream while the stream
is open, initialize its constraints as snd_pcm_open() does and fill
it in from the BE parameters once the mandatory machine level
be_hw_params_fixup() has run, so that the compressed FE lends a valid
and populated runtime just like a PCM FE does.

This also allows a BE to be shared between a compressed and a PCM
FE, which is needed to play a notification over PCM to an endpoint
which a compressed stream is already using.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Liam Girdwood <liam.r.girdwood@intel.com>
Link: https://patch.msgid.link/20261007084955.1256-6-peter.ujfalusi@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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