drm/amd/display: Remove unused variables in dcn21_hwseq.c

Fix the below compiler error:

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn21/dcn21_hwseq.c:229:11: error: unused variable 'otg_inst' [-Werror,-Wunused-variable]
        uint32_t otg_inst = pipe_ctx->stream_res.tg->inst;
                 ^
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn21/dcn21_hwseq.c:226:20: error: unused variable 'cmd' [-Werror,-Wunused-variable]
        union dmub_rb_cmd cmd;

Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Jerry Zuo <jerry.zuo@amd.com>
Cc: Yongqiang Sun <yongqiang.sun@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Yongqiang Sun <Yongqiang.Sun@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>