drm/dp_mst: Check if primary mstb is null

Unfortunately drm_dp_get_mst_branch_device which is called from both
drm_dp_mst_handle_down_rep and drm_dp_mst_handle_up_rep seem to rely
on that mgr->mst_primary is not NULL, which seem to be wrong as it can be
cleared with simultaneous mode set, if probing fails or in other case.
mgr->lock mutex doesn't protect against that as it might just get
assigned to NULL right before, not simultaneously.

There are currently bugs 107738, 108616 bugs which crash in
drm_dp_get_mst_branch_device, caused by this issue.

v2: Refactored the code, as it was nicely noticed.
    Fixed Bugzilla bug numbers(second was 108616, but not 108816)
    and added links.

[changed title and added stable cc]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108616
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107738
Link: https://patchwork.freedesktop.org/patch/msgid/20181109090012.24438-1-stanislav.lisovskiy@intel.com

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -1275,6 +1275,9 @@ static struct drm_dp_mst_branch *drm_dp_get_mst_branch_device(struct drm_dp_mst_
 	mutex_lock(&mgr->lock);
 	mstb = mgr->mst_primary;
 
+	if (!mstb)
+		goto out;
+
 	for (i = 0; i < lct - 1; i++) {
 		int shift = (i % 2) ? 0 : 4;
 		int port_num = (rad[i / 2] >> shift) & 0xf;