drm/edid: add a helper function to extract the speaker allocation data block (v3)

This adds a helper function to extract the speaker allocation
data block from the EDID.  This data block describes what speakers
are present on the display device.

v2: update per Ville Syrjälä's comments
v3: fix copy/paste typo in memory allocation

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Rafał Miłecki <zajec5@gmail.com>