drm/dp_mst: Check crc4 value while building sideband message

Check that computed crc value is matching the one encoded in the message.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200203121620.9002-1-benjamin.gaignard@st.com