drm/radeon/kms/ni: fix packet2 handling for VM IB parser

Packet2 is only one dword.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>