drm/edid: make drm_edid_to_eld() static

This is no longer needed outside of drm_edid.c.

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/9c0be7b97d0144ed0419c87ac42b30f5835ca7e6.1509545641.git.jani.nikula@intel.com