drm/edid: build ELD in drm_add_edid_modes()

Call drm_edid_to_eld() from drm_add_edid_modes() to fill in the ELD
automatically. There's no harm in doing this for connectors that do not
support audio.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/551b2e1cf19be04c510e7865d7539cfc2f54ea89.1509545641.git.jani.nikula@intel.com