drm/i915: add a common connector type independent destroy hook

Almost all of the connector destroy functions do the same thing. The
differences are in the edid, detect_edid and panel cleanups, but those
are safely NULL when not initialized. Roll out a common connector
destroy hook.

Inspired by commit bc3213c4 ("drm/i915: Drop the eDP check from
intel_dp_connector_destroy()").

Cc: 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/20181009141103.20387-1-jani.nikula@intel.com