drm/edid: abstract connector ELD clearing

Preparation for future work. No functional changes.

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/95e83e7a62566b65a56c8cb5038ddf0c6b8e5e95.1509545641.git.jani.nikula@intel.com