drm/mst: Avoid processing partially received up/down message transactions

drm/mst: Avoid processing partially received up/down message transactions

commit 636c4c3e762b62aa93632c645ca65879285b16e3 upstream.

Currently we may process up/down message transactions containing
uninitialized data. This can happen if there was an error during the
reception of any message in the transaction, but we happened to receive
the last message correctly with the end-of-message flag set.

To avoid this abort the reception of the transaction when the first
error is detected, rejecting any messages until a message with the
start-of-message flag is received (which will start a new transaction).
This is also what the DP 1.4 spec 2.11.8.2 calls for in this case.

In addtion this also prevents receiving bogus transactions without the
first message with the the start-of-message flag set.

v2:
- unchanged
v3:
- git add the part that actually skips messages after an error in
  drm_dp_sideband_msg_build()

Cc: Dave Airlie <airlied@redhat.com>
Cc: Lyude <lyude@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude <lyude@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170719134632.13366-1-imre.deak@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>