DRM: add help to get ELD speaker allocation

Add helper to allow users to retrieve the speaker allocations without
knowledge of the ELD structure.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>