aarch64: vp9mc: Simplify the extmla macro parameters

Fold the field lengths into the macro.

This makes the macro invocations much more readable, when the
lines are shorter.

This also makes it easier to use only half the registers within
the macro.

This is cherrypicked from libav commit
5e0c2158.

Signed-off-by: Martin Storsjö <martin@martin.st>