drm/dp_mst: Remove bogus conditional in drm_dp_update_payload_part1()

There's no reason we need this, it's just confusing looking.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Juston Li <juston.li@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181214012604.13746-2-lyude@redhat.com