target/mips: Remove CPUMIPSState* argument from gen_msa*() methods

The gen_msa*() methods don't use the "CPUMIPSState *env"
argument. Remove it to simplify.

Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20201208003702.4088927-9-f4bug@amsat.org>