drm/i915/audio: constify ELD pointers

The hooks aren't supposed to modify the ELD, so use const pointer. As a
drive-by fix, use drm_eld_size() to log ELD size.

Suggested-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180619124437.10982-1-jani.nikula@intel.com