ASoC: Intel: Skylake: Add support for virtual dsp widgets

In SKL topology routes, some paths can be connected by a widget
which are not a DSP FW widget and virtual with respect to
firmware. In these case when module has to bind, then the
virtual DSP modules needs to skipped till a actual DSP module is
found which connects the pipelines.

So we need to walk the graph and find a widget which is real in
nature. This patch adds that support and splits
skl_tplg_pga_dapm_pre_pmu_event() fn with parsing code to
skl_tplg_bind_sinks() fn and call that recursively as well as
while parsing

The patch moves code a bit while splitting so diffstat doesn't
tell real picture

Signed-off-by: Jeeja KP <jeeja.kp@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>