target/arm: Fix decode for VDOT (indexed)

We were extracting the M register twice, once incorrectly
as M:vm and once correctly as rm.  Remove the incorrect
name and remove the incorrect decode.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210525010358.152808-87-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>