freedreno/ir3: fix UMAD
Looks like none of the mad variants do u16 * u16 + u32, so just add in the extra value "by hand". Signed-off-by:Ilia Mirkin <imirkin@alum.mit.edu> Cc: "10.3 10.4" <mesa-stable@lists.freedesktop.org> (cherry picked from commit de83ef67)