drm/dp_mst: Fix NULL deref in drm_dp_get_one_sb_msg()

While we don't need this function to store an mstb anywhere for UP
requests since we process them asynchronously, we do need to make sure
that we don't try to write to **mstb for UP requests otherwise we'll
cause a NULL pointer deref:

RIP: 0010:drm_dp_get_one_sb_msg+0x4b/0x460 [drm_kms_helper]
Call Trace:
? vprintk_emit+0x16a/0x230
? drm_dp_mst_hpd_irq+0x133/0x1010 [drm_kms_helper]
drm_dp_mst_hpd_irq+0x133/0x1010 [drm_kms_helper]
? __drm_dbg+0x87/0x90 [drm]
? intel_dp_hpd_pulse+0x24b/0x400 [i915]
intel_dp_hpd_pulse+0x24b/0x400 [i915]
i915_digport_work_func+0xd6/0x160 [i915]
process_one_work+0x1a9/0x370
worker_thread+0x4d/0x3a0
kthread+0xf9/0x130
? process_one_work+0x370/0x370
? kthread_park+0x90/0x90
ret_from_fork+0x35/0x40

So, fix this.

Signed-off-by: Lyude Paul <[e-mail]>
Fixes: fbc821c4a506 ("drm/mst: Support simultaneous down replies")
Cc: Wayne Lin <[e-mail]>
Cc: Lyude Paul <[e-mail]>
Cc: Wayne Lin <[e-mail]>
Cc: Sean Paul <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Sean Paul <[e-mail]>

drivers/gpu/drm/drm_dp_mst_topology.c

···
 	int basereg = up ? DP_SIDEBAND_MSG_UP_REQ_BASE :
 			   DP_SIDEBAND_MSG_DOWN_REP_BASE;
 
-	*mstb = NULL;
+	if (!up)
+		*mstb = NULL;
 	*seqno = -1;
 
 	len = min(mgr->max_dpcd_transaction_bytes, 16);