drm/dp-mst-helper: Don't use uninitialized fields of the sideband message header

We could be using uninitialized fields of the header in
drm_dp_encode_sideband_msg_hdr(), for instance hdr->somt is set to 1 in
the first patcket but never set to 0 otherwise.

Always clear the header at the start then.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>