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 <[email protected]>
Suggested-by: Emil Velikov <[email protected]>
Suggested-by: Lyude Paul <[email protected]>
Signed-off-by: Lyude Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Lyude Paul <[email protected]>

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