ASoC: Intel: Remove sst_dsp_get_thread_context

ASoC: Intel: Remove sst_dsp_get_thread_context

While sst_dsp_get_thread_context() is declared as solution-agnostic, it
is only used by /skylake/ solution. Majority of thread_context field
usages are direct accesses. Improve code cohesiveness and convert to
single usage model.

Signed-off-by: Cezary Rojewski <cezary.rojewski@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Liam Girdwood <liam.r.girdwood@intel.com>
Link: https://lore.kernel.org/r/20201006064907.16277-14-cezary.rojewski@intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>