drm: Add drm_dp_destroy_connector helper and use it

drm: Add drm_dp_destroy_connector helper and use it

drm_dp_mst_topology_mgr_cbs.destroy_connector callbacks are identical
amongst every driver and don't do anything other than cleaning up the
connector (drm_connector_unregister()/drm_connector_put()) except for
amdgpu_dm driver where some amdgpu_dm specific code in there which I
an not sure if it should stay or not.

Create and use a helper which calls driver's destroy_connector hook if
available otherwise does cleanup internally.

This is the step towards removing identical hooks from every driver.

Signed-off-by: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@intel.com>
Suggested-by: Emil Velikov <emil.velikov@collabora.com>
Suggested-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200307083023.76498-5-pankaj.laxminarayan.bharadiya@intel.com
Reviewed-by: Lyude Paul <lyude@redhat.com>