x86: fix Disp8 handling for scalar AVX512_4FMAPS insns

x86: fix Disp8 handling for scalar AVX512_4FMAPS insns

Just like their packed counterparts the memory operand is always 16
bytes wide, and the Disp8 scaling is the same for all of them. (As a
side note: I'm also surprised by there being AVX512VL variants of
these as well as the AVX512_4VNNIW ones - the SDM doesn't define any
such.)

Adjust the test cases also for the packed forms to actually live up to
their promise of testing correct Disp8 encoding.