drm/dp/mst: Get validated port ref in drm_dp_update_payload_part1()

We can thank KASAN for finding this, otherwise I probably would have spent
hours on it. This fixes a somewhat harder to trigger kernel panic, occuring
while enabling MST where the port we were currently updating the payload on
would have all of it's refs dropped before we finished what we were doing:

==================================================================
BUG: KASAN: use-after-free in drm_dp_update_payload_part1+0xb3f/0xdb0 [drm_kms_helper] at addr ffff8800d29de018
Read of size 4 by task Xorg/973
=============================================================================
BUG kmalloc-2048 (Tainted: G    B   W      ): kasan: bad access detected
-----------------------------------------------------------------------------

INFO: Allocated in drm_dp_add_port+0x1aa/0x1ed0 [drm_kms_helper] age=16477 cpu=0 pid=2175
	___slab_alloc+0x472/0x490
	__slab_alloc+0x20/0x40
	kmem_cache_alloc_trace+0x151/0x190
	drm_dp_add_port+0x1aa/0x1ed0 [drm_kms_helper]
	drm_dp_send_link_address+0x526/0x960 [drm_kms_helper]
	drm_dp_check_and_send_link_address+0x1ac/0x210 [drm_kms_helper]
	drm_dp_mst_link_probe_work+0x77/0xd0 [drm_kms_helper]
	process_one_work+0x562/0x1350
	worker_thread+0xd9/0x1390
	kthread+0x1c5/0x260
	ret_from_fork+0x22/0x40
INFO: Freed in drm_dp_free_mst_port+0x50/0x60 [drm_kms_helper] age=7521 cpu=0 pid=2175
	__slab_free+0x17f/0x2d0
	kfree+0x169/0x180
	drm_dp_free_mst_port+0x50/0x60 [drm_kms_helper]
	drm_dp_destroy_connector_work+0x2b8/0x490 [drm_kms_helper]
	process_one_work+0x562/0x1350
	worker_thread+0xd9/0x1390
	kthread+0x1c5/0x260
	ret_from_fork+0x22/0x40

which on this T460s, would eventually lead to kernel panics in somewhat
random places later in intel_mst_enable_dp() if we got lucky enough.

Signed-off-by: Lyude <cpaul@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>