drm/dp-mst: Remove branches before dropping the reference

When we unplug a dp mst branch we unreference the entire tree from
the root towards the leaves. Which is ok, since that's the way the
pointers and so also the refcounts go.

But when we drop the reference we must make sure that we remove the
branches/ports from the lists/pointers before dropping the reference.
Otherwise the get_validated functions will still return it instead
of returning NULL (which indicates a potentially on-going unplug).

The mst branch destroy gets this right for ports: First it deletes
the port from the ports list, then it unrefs. But the ports destroy
function gets it wrong: First it unrefs, then it drops the ref. Which
means a zombie mst branch can still be validate with get_validated_mstb_ref
when it shouldn't.

Fix this.

This should address a backtrace Dave dug out somewhere on unplug:

 [<ffffffffa00cc262>] drm_dp_mst_get_validated_mstb_ref_locked+0x92/0xa0 [drm_kms_helper]
 [<ffffffffa00cc211>] drm_dp_mst_get_validated_mstb_ref_locked+0x41/0xa0 [drm_kms_helper]
 [<ffffffffa00cc2aa>] drm_dp_get_validated_mstb_ref+0x3a/0x60 [drm_kms_helper]
 [<ffffffffa00cc2fb>] drm_dp_payload_send_msg.isra.14+0x2b/0x100 [drm_kms_helper]
 [<ffffffffa00cc547>] drm_dp_update_payload_part1+0x177/0x360 [drm_kms_helper]
 [<ffffffffa015c52e>] intel_mst_disable_dp+0x3e/0x80 [i915]
 [<ffffffffa013d60b>] haswell_crtc_disable+0x1cb/0x340 [i915]
 [<ffffffffa0136739>] intel_crtc_control+0x49/0x100 [i915]
 [<ffffffffa0136857>] intel_crtc_update_dpms+0x67/0x80 [i915]
 [<ffffffffa013fa59>] intel_connector_dpms+0x59/0x70 [i915]
 [<ffffffffa015c752>] intel_dp_destroy_mst_connector+0x32/0xc0 [i915]
 [<ffffffffa00cb44b>] drm_dp_destroy_port+0x6b/0xa0 [drm_kms_helper]
 [<ffffffffa00cb588>] drm_dp_destroy_mst_branch_device+0x108/0x130 [drm_kms_helper]
 [<ffffffffa00cb3cd>] drm_dp_port_teardown_pdt+0x3d/0x50 [drm_kms_helper]
 [<ffffffffa00cdb79>] drm_dp_mst_handle_up_req+0x499/0x540 [drm_kms_helper]
 [<ffffffff810d9ead>] ? trace_hardirqs_on_caller+0x15d/0x200 [<ffffffffa00cdc73>]
 drm_dp_mst_hpd_irq+0x53/0xa00 [drm_kms_helper] [<ffffffffa00c7dfb>]
 ? drm_dp_dpcd_read+0x1b/0x20 [drm_kms_helper] [<ffffffffa0153ed8>]
 ? intel_dp_dpcd_read_wake+0x38/0x70 [i915] [<ffffffffa015a225>]
 intel_dp_check_mst_status+0xb5/0x250 [i915] [<ffffffffa015ac71>]
 intel_dp_hpd_pulse+0x181/0x210 [i915] [<ffffffffa01104f6>]
 i915_digport_work_func+0x96/0x120 [i915]

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>