ASoC: dpcm: rework runtime stream merge

The goal of this patch is to simplify a bit dpcm runtime stream merge
by removing several local variables.

ATM, merge functions return the BE 'filter' values which should then be
filtered against the FE stream values. This create a lot of local
variable and unnecessary init of min and max.

Instead of this, we can pass the FE stream values directly and let the
BE filtering functions perform the merge 'in-place'

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>