[AMDGPU] Disassembler: fix s_buffer_store_dword instructions

Summary: s_buffer_store_dword instructions sdata operand was called sdst in encoding. This caused disassembler to fail.

Reviewers: tstellarAMD, vpykhtin, artem.tamazov

Subscribers: arsenm, nhaehnle, rampitec

Differential Revision: https://reviews.llvm.org/D27100

llvm-svn: 288657