x86: restore masking of displacement kinds

Commit 7d5e4556 rendered the check near the end of what is now
i386_finalize_displacement() entirely dead for AT&T mode, since for
operands involving a displacement .unspecified will always be set. But
the logic there is bogus anyway - Intel syntax operand size specifiers
are of no interest there either. The only thing which matters in the
"displacement only" determination is .baseindex.

Of course when masking displacement kinds we should not at the same time
also mask off other attributes.

Furthermore the type mask returned by lex_got() also needs to be
adjusted: The only case where we want Disp32 (rather than Disp32S) is
when dealing with 32-bit addressing mode in 64-bit code.