drm/mst: Separate sideband packet header parsing from message building

In preparation of per-branch device down message handling, separate
header parsing from message building. This will allow us to peek at
figure out which branch device the message is from before starting to
parse the message contents.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Wayne Lin <waynelin@amd.com>
[seanpaul s/drm_dp_sideband_msg_build/drm_dp_sideband_msg_set_header/]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200213211523.156998-2-sean@poorly.run