drm/dp/mst: Reverse order of MST enable and clearing VC payload table.

On DELL U3014 if you clear the table before enabling MST it sometimes
hangs the receiver.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Cc: stable@vger.kernel.org
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>