KVM: x86 emulator: Stop passing ctxt->ops as arg of decode helpers

Dereference it in the actual users: only do_insn_fetch_byte().

This is consistent with the way __linearize() dereferences it.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>