Spec says[1] Allocated_PBN is 16 bits

[1]- DisplayPort 1.2 Spec, Section 2.11.9.8, Table 2-98

Fixes: ad7f8a1f ("drm/helper: add Displayport multi-stream helper (v0.6)")
Cc: Lyude Paul <lyude@redhat.com>
Cc: Todd Previte <tprevite@gmail.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190829165223.129662-1-sean@poorly.run